Domains are domains, as you can see in the panel.
First of all, it’s not a “free trial”. The services are fundamentally different that you cannot use one to evaluate another.
For subdomains, the domains that shows up only in this interface are the common subdomains that iFastNet enables across the entire reseller system, while those shows up in both places are domains that InfinityFree owns.
For custom domains there’s a handy article How to add your own domain to your account.
Like how you would use the other domain.
Like how the other domain works. There’s not really something different than you would imagine.
The subdomains you added after the account creation will have a different folder for it, if you really want a difference to keep in mind.