Alright, after looking over the template (and with a little help from chatgpt), the way to configure a plugin is:

mariadb:
    plugin-maturity: stable
    plugin-load-add:
      - 'auth_ed25519=auth_ed25519.so'
      - 'disks=disks.so'
      - 'server_audit=server_audit2.so'
      - 'simple_password_check=simple_password_check.so'
      - 'xxxxxx=xxxxx.so'

which results in:

[mariadb]
plugin-load-add = auth_ed25519=auth_ed25519.so
plugin-load-add = disks=disks.so
plugin-load-add = server_audit=server_audit2.so
plugin-load-add = simple_password_check=simple_password_check.so
plugin-load-add = xxxxxx=xxxxx.so