Is it possible to use a infinityfree domain as my minecraft server domain?

You can host your main website with InfinityFree, if that’s what you want, and have a subdomain pointed to Aternos, or if you want your domain to function as a Minecraft server only, you can follow Aternos’ tutorial:

Keep in mind that if you also want to host a site with InfinityFree, you’ll need to then set up Cloudflare: