Npub is an encoded pubkey

nak decode npub

Will give it to you.

Notice the characters in your npub and taproot address are the same module prefix and checksum.

To spend just use your privkey.  Create a tx and sign with schnorr.  Sparrow wallet is a great example.

I'll write a web wallet at some point.  But I expect you know how to do this, p2tr 

 I'll try when I come back home, currently away on vacations, just shitposting