There is not a bespoke setup that you need to implement atproto. In fact, there are already a variety of applications making use of it (some to a higher degree than others). There are community implementations of app views, relays, PLC directories, and PDSes already in the wild, and - although I admittedly have a biased ear on the conversation - developers tend to appreciate the _lack_ of complication when implementing things.
Yes, it is absolutely possible to build on atprotocol and be a part of the network. We have written a good bit about this already here: https://atproto.com/articles/atproto-for-distsys-engineers (in addition to the recently updated documentation in general)
The Bluesky app and the protocol it is built on are open source, MIT licensed.
The app is built using React/React Native as well as a variety of other open source tools. We maintain multiple React Native libraries as well that are open source.
The network is open, and anyone has access to the firehose. Third parties are free to run labelers that integrate with the application as well as build feeds which can be used by yourself or others inside the app. Anyone is free to use the open source SDKs to build their own third-party Bluesky apps or entirely unique applications using atprotocol.
To be frank, "it's just an app that happens to be open source" seems to be a pretty bad faith take on what Bluesky is.