You get all the code pertaining to the classic (self-hosted) instance of the platform, yes. Please note, you are currently trialing cloud, so there are some cloud only features the classic platform doesnt have, such as live community (seeing who is online instantly and who is typing etc), courses, live topics and a few other bits. You can take a look on our main purchase pages to see what each contains
https://invisioncommunity.com/buy
As far as how you can build on the platform, take a look at the documentation here, which will give you a clearer indication
https://invisioncommunity.com/developers/
Our EULA can be found on the following page, along with any other agreements you may be interested in
https://invisioncommunity.com/legal/