Have you heard of Amber? It's a mobile app which stores your nsec to sign in various apps (not all support amber login)
You can get it on nostr:nprofile1qqs83nn04fezvsu89p8xg7axjwye2u67errat3dx2um725fs7qnrqlgpzdmhxue69uhhqatjwpkx2urpvuhx2ue0e2p4dj or if you don't use zapstore directly from github https://github.com/greenart7c3/amber