GythaOgg pretty much said it all.
I have never owned one but two of my best friends do, and have, for years. One shows hers (but doesn’t breed them.)
Bloat is a big concern. Look it up and have a bloat kit on hand, understand what the signs are!
They are fairly low-energy, you do not need a big yard as long as you commit to regular walkies and exercise. Most dogs don’t take advantage of a big yard anyway - I have four dogs, all fairly high-energy, and a humungous yard. But they go out there, pee, poop, play for maybe two minutes, then want to be back inside where I am. They basically don’t get exercise in the yard - I have to provide that because I am the Provider of All Things Fun and Good.
Since this woman with the puppies isn’t doing it right, be aware that Danes are prone to joint problems, skin problems/allergies, deafness (don’t choose a puppy that is mostly white with some merle, or who has a white face and ears) along with bloat. Also they can be very fussy eaters. The two Dane people I know feed raw, not kibble/pellets because the latter simply didn’t work well.
Funny story: I took one of my dogs to an agility trial (she wasn’t competing yet) just to get her out and about. She was just coming out of her first heat. She is a smallish dog. Met a friend there who was with her Great Dane, an intact male who was competing that day. I was sitting on the ground with my small, apparently extremely attractively-smelling girl dog when my friend’s mostly impeccably-trained infatuated 150 lb dog hiked his leg. On my back. I was absolutely drenched with Great Dane urine. A call was put out for dry tee shirts, baby wipes, towels, etc and much hilarity ensued.
Everything is bigger with big dogs, including vet bills, general maintenance (fleas, HW protection), pee and slobber production…