On the technical side, Nostr auth could work side by side (stacker.news - link to your Nostr account), or it can replace it (don't have an example aside from nostr apps). If nostr is your main identity system it may or may not affect your traditional model - you could just auth all HTTP API calls to traditional backend, or you could do a full-blown nostr app that uses nostr events and relays and no backend. I tried to briefly cover all this on nostrlogin.org but maybe it wasn't all that clear.