SWAG is great for overwhelmed Nginx beginners. It comes preconfigured with reasonable defaults and also provides configs for a bunch of popular services: github.com/linuxserver/reverse-proxy-confs. Both Bitwarden and Vaultwarden are on there.
Note that this setup assumes that you will run your service (Bitwarden/Vaultwarden) in a Docker container. You can make SWAG work with something that’s running directly on the host, but I’d recommend not starting with that until you’ve fooled around with this container setup a bit and gained a better understanding of how Nginx and reverse proxies in general work.