This is where things get kind of blurry for me since I can see all of the steps on their website but don't really understand all of the code in their instructions.
Most
code command on link i mentioned is about setting the hardware, system and BTCPayServer itself. You should able to find exaplanation or example of most command from google search result.
I am sure that the node could be used for all sorts of queries but I just have no idea how to do all of that...yet.
Assuming you already know how to obtain data from API, you just need to read Bitcoin Core RPC[1] and BTCPay GreenField API[2] documentation.
[1]
https://developer.bitcoin.org/reference/rpc/[2]
https://docs.btcpayserver.org/Development/GreenFieldExample/