Learn how to configure your Proxmox server to allow you to use your Yubikey as an MFA / 2FA device, using the WebAuthn protocol.
Generate your API key for your Yubikey.
Link your Yubikey with GPG
Change the mode of your Yubikey so that you can use it for PGP/GPG as well.
See how you can use your Yubikey to SSH into remote servers using a pin code.
A cheatsheet for yubikey.