In the most unintuitive ways possible.
I have a perfect png signature with transparency, but there's no way to import it.
https://support.apple.com/guide/preview/annotate-a-pdf-prvw1...
Page addition: drag a page from another PDF sidebar into the Preview sidebar
Merging: same, but for a whole PDF
Encryption: File > Edit Permissions > Require Password to Open Document
Decryption: If you have the password, then the same flow
Redaction/Markup/Signatures: All from the Markup menu