I used to have a job involving going into proper mansions, and in the centre of London (Chelsea, Belgravia, Mayfair) so these were sometimes worth £20m+ and that was a decade or so ago
Fascinating. Heated garages for car collections, prep kitchens behind the real kitchens yes, tiny windowless maid's rooms sometimes, once a hidden underground swimming pool in the middle of Chelsea, once a whole subbasement for security staff (a royal family). Endless guest suites, hand painted wallpaper etc. Incredible art and occasionally a paint by numbers type thing mixed in by someone with a hobby.
And some relatively normal period homes, just quite fancy and very well located. Very little outside space of course, mostly small roof terraces.
However, money most definitely does not buy you taste (unless you hire a designer, which lots did). Also, some were a complete mess.
Bear in mind all the owners would have had other homes too - the more expensive the house the more likely it's the 3rd residence!
On size, 5,000 sq ft is massive for an urban house. 30,000 sq ft is a large embassy or office, but the old ones did used to be houses when first built. Watch Bridgerton or similar for a sense of the scale.