No, InfinityFree’s subdomains can only be used on InfinityFree, or on iFastNet’s paid hosting if you have them migrate your account. They cannot be used with other servers.
There are, however, services that provide free subdomains that you can point to your own server, such as No-IP, FreeDNS, and Duck DNS.
No, that only applies if you’re using your own custom domain, but I can see how that could be unclear.