Tips on reactive puppy in apartment

Hey everyone, I posted this on r/goldenretrievers so I hope its okay I post it here too because I'm really looking for some advice.

We just got a puppy a few weeks ago, shes 11 weeks old and doing great. She's adjusted well and is learning our routine but still has SO much to learn in terms of training. I want her to be a great dog but at this stage in her life she gets excited by everything.

I'm currently in an apartment over 10 floors up. I don't intend to live here forever but there's tons of green space and we walk all the time. The problem is, we live in a building with lots of other dogs and people constantly using the elevator, and she has to go out all the time during her potty training phase.

I train her often, but she is still so young and has such a small attention span. I want her to be non-reactive when other people step in the elevator (or even worse, other dogs) but it just seems hopeless because it's so much stimulation for her all at once and she forgets I exist. I have to get on my knees and hold her back so she doesn't try to play with everyone she meets.

Is there any any way I can work on this with her? She already knows a lot of tricks so she is good with simple training, but these situations just feel so uncontrolled and I don't want her to develop bad habits. Or, do I just have to deal with these encounters and wait until she's a little older to start reinforcing non-reactivity with her?

Thank you all for the advice.