Your question is not clear enough, but I’ll provide some information about SSL certificate installation and DNS data.
Infinityfree does provide free SSL certificates for 3 months, and you can renew those certificates after that too without any cost.
Talking about SSL certificate installation, it’s pretty simple in here to install. If you follow this link, you’ll be redirected to the SSL installation page. Login to your account - InfinityFree
Or you can follow the below article.
Main Domain vs. Subdomain:
In the context of SSL, both main domains and subdomains can have SSL certificates. Subdomains, like raggedyedge.free.nf, can also have SSL, but sometimes they might be covered by a wildcard SSL certificate that covers all subdomains.
DNS data (Domain Name System)
If you are concerned about DNS data and why you should do it, it depends on the task that you want to achieve. Imagine you want your website to be accessible via WWW, In that case, where those DNS come into play, you’ll basically have to create an A record to point your website’s IP address to a WWW subdomain. And without DNS data, you cannot access your site with the domain name. It’ll be something like this: 168.143.43.2.
For more details about DNS, please visit this website and read about it.
https://www.cloudflare.com/learning/dns/what-is-dns/
And lastly, about redirecting your website to a different domain or pointing it to a different page, it depends on your expectations.
I hope this might be helpful for you; at least that’s what I understand about your question.