• Real estate entrepreneur Grant Cardone, who has a gallery in his Miami home, said collecting art gives him more fulfillment than investing in the stock market.
• Christian Levett, a former investment manager, opens his Florence home for private tours of his all-female art collection.
• The Boros Collection houses contemporary art in a vast World War II bunker in Berlin — and owners Christian and Karen Boros live in a penthouse apartment above it.

Multimillionaire Grant Cardone, who has been collecting art for around 15 years, says he's a spontaneous buyer.

"I don't consider myself a connoisseur. I'm very new to the art world. If I like it, I buy it. I don't care who did it," he told CNBC. Alongside pieces displayed throughout his home, Cardone also has an art gallery to house his considerable collection.

CNBC spoke to Cardone by video call — behind him in his Miami home office was an untitled piece by American graffiti artist Retna that Cardone bought in an online auction.

"I clicked the button — really hadn't done any research … and got the piece … And it got here and I absolutely freaking loved it," he said. He paid "maybe $140,000" for the work, he said.

Along a corridor in Cardone's home are two pieces by American pop artist Burton Morris, both depicting red Coca-Cola bottles lined up in a repeating pattern named Coca-Cola 50A and Coca-Cola 50B. "This I bought from Tommy Hilfiger … it reminds me of the importance of scaling," Cardone said — fashion designer Hilfiger is the home's previous owner.

Cardone, a real estate investor and author of "The 10 X Rule: The Only Difference Between Success and Failure," has around 17 million followers on social media and uses his platforms to give occasional advice on art investing.

"[Followers are] starting to see the art saying, hey, you know, [has] that been good for you? And I'm like, yeah, it's good for me … It's better than the dollar or the euro … The stock market doesn't give me any fulfillment, I don't go back and look at my Apple shares and feel good about it. But I walk in my gallery or down the kitchen or in my office and I see a piece and I'm like, man, it's super cool."

Inside Cardone's gallery — complete with floor-to-ceiling windows and a security guard — is a work by American contemporary artist Kenny Scharf titled "Blipsibshabshok" (1997), an abstract painting featuring colorful futuristic symbols. Cardone owns a second Scharf, "Controlopuss" (2018), a striking image of a red multi-legged creature, acquiring it for $279,400 from auction house Phillips.

"This is a Basquiat right here. The original would be $45 million," Cardone said, pointing to a print of a Jean-Michel Basquiat piece titled "Flexible" (1984/2016). The original was sold by auction house Phillips for $45.3 million in 2018. "This piece I bought with the house," he said, gesturing to a work above the Basquiat titled "Read More" by American contemporary artist Al-Baseer Holly.

Grant said he chooses pieces to buy on instinct. "I'll try to walk away from it. And if I keep seeing it, or I keep thinking about it, then I go back and say, OK, I'm supposed to have this," he said.

"I plan on never selling any this stuff. It's really for my personal enjoyment. And you know, art makes me happy," he said.

Female art in Florence

Former investment banker Christian Levett has a different approach. He's been collecting art for almost 30 years, starting with old master paintings and Roman, Greek and Egyptian antiquities before moving on to pieces by female abstract expressionists.

Aside from owning an art museum in Mougins, France, Levett conducts tours of the artwork on the walls of his home in Florence, Italy, where he lives for six months of the year — you could say his whole home is an art gallery. "It's, kind of, a museum by private tour," Levett told CNBC by phone.

Close to the city's famous Ponte Vecchio bridge, Levett's home has 20-foot-high ceilings, original frescoes and two floors of art, all by women. The collection is largely made up of abstract expressionist works by artists such as Elaine de Kooning, Pat Passlof and Helen Frankenthaler.

Once or twice a week, Levett invites small groups to see his collection, often taking tours himself. Groups are sometimes made up of students from the American colleges that have outposts in Florence, such as Harvard University and New York University, or come from museum or patrons' groups.

A 1977 painting by American artist Joan Mitchell is a highlight of Levett's collection, he said. The large piece, titled "When They Were Gone," is nearly 240cm tall and 180cm wide, and hangs in his dining room.

Levett acquired it for about $2.8 million around 2015.

"It's now probably a $15 million to $18 million picture at auction … Mitchell has always been one of the most important female painters of the 20^th century," Levett said.

He also spoke highly of an Elaine de Kooning oil painting of John F. Kennedy, commissioned as part of a series of portraits of the former U.S. president in 1963. Levett bought the artwork in 2020, paying around $600,000.

Levett said he opens his home to students in part because doing so might spark an interest in supporting art in future. "The students ... are the acorns of the art world," he said.

Work by female artists is Levett's focus, and he is set to re-open his museum in France as the Female Artists Mougin Museum on June 21. He is currently selling the museum's previous collection of art and antiquities via a series of sales at London auction house Christie's, which have reached almost £9.5 million ($11.9 million) so far.

Bunker art

At a unique art space in Berlin, husband and wife Christian and Karen Boros live in a 6,000 square foot penthouse apartment above their private collection. The Boros Collection is housed in a former World War II bunker, a vast, high-rise building the couple acquired in 2003 and spent several years converting into a five-floor exhibition space, with their home on the sixth.

The bunker sheltered up to 4,000 people during the war, after which it was used as a storage facility for tropical fruit before becoming a nightclub. According to Raoul Zoellner, director of the Boros Foundation, 450 tons of concrete ceilings and walls were removed during its conversion into an exhibition space and home.

Christian, an advertising entrepreneur, bought his first artwork — a spade by German artist Joseph Beuys — when he was 18, he told the Financial Times.

"The bunker is not a museum … but an exceptional project initiated by an enthusiastic collector couple who could not have imagined how many diamond saws it would take to tear down dozens of bunker walls — or what that would set in motion," Zoellner said in an emailed statement.

Nearly 600,000 people have taken guided tours of the bunker since its conversion in 2008, with pieces from the Boros Collection shown on rotation, Zoellner added. At the moment, there are 114 works on view, with a "focus on the human body in a multiplicity of positions," Zoellner said. "The works home in on the constant compulsion to optimize, the gradual adaptation of our bodies to technological devices," he said.

Correction: An earlier version of this story incorrectly named the artists who were part of the abstract expressionist movement.