Which ones specifically? I don't know of any popular Nostr web apps that don't allow for signing in using a browser extension these days. Native mobile apps are another matter. Android has Amber signing app for those, but there is not currently any signer application available on iOS for native apps. There is a browser extension available for Safari called NoStore, I believe, and nsec.app, which is available for all platforms, but both of those only work with web apps and not with native apps.