I've recently recorded technical case study that goes through whole BTCPay setup... that can be used to host multiple merchants.

nostr:note1a4fxsswej4c3ng708vt60d2j5e55mkauxxtfya0wy0l9wtpm5glq5rmhtc

See if that guide works for you; I also highly recommend videos by nostr:npub1rxysxnjkhrmqd3ey73dp9n5y5yvyzcs64acc9g0k2epcpwwyya4spvhnp8

https://www.youtube.com/watch?v=KqsM-n-e4aY

And then nostr:npub15klkdfx9sh3y096a5jf895rcvkmkzvly2fm9dgxq6huqhp9576jsav4m73 has nice BTCPay guides as well:

https://www.youtube.com/watch?v=F-2tDH9J-_s