How do you deal with stimulation/exercise when you live in an apartment and puppies can’t be outside?

Pretty much the title. I am getting my first puppy (grew up with dogs, just recently am positioned to finally have my own!), what do people do to ensure the pup is exercised/stimulated enough to not be a complete maniac while also ensuring their safety?

I have found a puppy kindergarten class and puppy social's through a training school in my area that has a space for puppies before they are fully vaccinated, but I feel like that will be twice a week at most? What do I do the rest of the time for them? Just mental stimulation?

The pup I am getting is a medium sized full blown mutt. I am assuming as an adult they will also be medium energy, I am under no illusion that will be the case for the first two years.

I am open to any other advise from fellow apartment dwellers that have dogs!