How do I use a specific SSH key for exe.dev? How do I manage multiple exe.dev accounts?

If you want to specify which key to use, use ssh -i ~/.ssh/id_ed25519_exe exe.dev or add the following stanza to your ~/.ssh/config:

Host exe.dev *.exe.xyz
  IdentitiesOnly yes
  IdentityFile ~/.ssh/id_ed25519_exe

If you have multiple accounts, you will need different SSH keys per account. You can create more keys with ssh-keygen -t ed25519 -f ~/.ssh/my-alt-account or similar. If you work on multiple accounts frequently, use a script like exe-ssh-config-generator to configure your SSH configuration to use the appropriate key for the appropriate hostnames.