Microsoft.Activities.Extension package is missing

what should I do if I have Studio 23.4 & .Net 5.0.17 and keep Windows as the compatibility instead of Legacy and still be able to Add to Dictionary, whether it’s downloading some other pkg or using VB code in an invoke code or what? I’m going through developer foundation training again to see what all has changed in the past several years and I can’t complete the exercises because everything is newer, and I need to learn the newer ways because what if a client has 22.x or 23.x and using .Net 5 or 6? How can you add a new key, value pair to an Dictionary in a for each type loop?? Thanks