THIS VIDEO is a good start. Simply drag and drop them from inside the Assets browser or append them from AAA_Nodes_X_X_X.blend file via File/Append/NodeTree/AAA and start using them as you would use any other geometry nodes. You can also use them as Blender Assets after you define the correct Assets path. However, if you append them as suggested previously, they'll automatically become Blender Assets inside your current file. After that, you can simply Shift+A/Group and pick the AAA node group you need.

Pro - for one (1) user
Pro Studio - for up to five (2-5) users

This really depends on various factors so make sure that you investigate how geometry nodes affect computer performance. The more geometry nodes you have in your node tree setup, the more processing will be needed. AAA Nodes have been tested on an old ASUS ROG laptop that has Intel Core i7-4710HQ CPU (@ 2.50GHz, 2.50 GHz) and 16GB of RAM.

Maybe. To be honest, atm there's no need for an addon. AAA Nodes are at the same time Assets and Geometry Nodes, which means that, once you define your Assets path, additional addon installation would only complicate things. But, we'll probably deliver AAA Nodes as an addon as well, jus to see what fits our users better and then make a final decision about how to deliver the package in the future.