Like others have said, usually there’s a bolt on the underside, but I have trouble seeing it in this picture. In some faucets (also some Grohe) there’s an Allen inside the faucet. Remove the spout by loosening the ring around the spout and there should be a threaded rod in which fits an Allen key.
The transfer into proton is indeed very easy and with their family pack you also get Mail, Drive, VPN, Calendar and they keep working on adding more. Very happy with proton and their fight for privacy