2 domain names – same content- absolute urls??

2.8.4
I hope to take this site live soon.
I realize i have to change all the ip#~username urls to the real domain.org name.
Still unclear about absolute urls and pointing the .com domain name to site??

Currently dev site using ip#~username/
This will be changed to https://www.realsite.org when going live.
This site uses WP as CMS. All the links on the site currently are absolute.

I would like to change the nameservers for https://www.realsite.COM to also point to and access https://www.realsite.org.(exact same content)
If the links on the .org site are absolute, it doesn’t seem like anything would work for the .com site??

I am sorry for not asking the question better.
It’s my lack of understanding as to how WP handles linkage,
as usually I build sites with mostly relative links and when pointing another domain to it all works.

Thanks
~bobbi