Domain.com and WWW.domain.com are different?

[Edit from the topic] https://www.blackhatworld.com/seo/u...ain-com-please-help-me.1188554/#post-12741294
(Because the previous topic has some errors that I can't edit. So I just created a new topic. I don't know if this violates the rules on BHW. If so I will learn from experience next time)
Hi everyone,
I'm using an expired domain. I have checked the backlink and most of them are linked to http://www.domain.com(instead of: https://domain.com)
My website currently is https:domain.com.(Non-www) and there were some posts and pages on it.
I'm recovering some old articles so how I can use 100% "POWER" of backlinks.
Questions:
1. http://www.domain.com and https://domain.com can be called 2 separate domains?
2. I have used the plugin: Really Simple SSL to redirect from http -> https so has the power of backlinks been changed or not?
3. For example, I need to recover a page: http://www.domain.com/page-abc to https//:domain.com/page-abc: How should I do so that I can get to the highest efficiency
My idea: I will create a sub-domain: https//:www.domain.com, then direct this sub-domain to my current domain (https//:domain.com). Details: Homepage to Homepage. Pages to Pages. What do you think about this? Any advice for me!
Thanks in advance!
Good day!

Do 301, No much different, the authority will pass.

Do a 301 redirect through htaccess file. that should let the authority pass.

nirose said:

Do a 301 redirect through htaccess file. that should let the authority pass.

Click to expand...

You mean, I should keep the current domain(https://domain.com) and make a 301 redirect from the old domain (http://www.domain.com) to https://domain.com through htaccess file. Right?
So by using, Can I direct Pages to Pages? Or Only Homepage?

I was about to say a 301 redirect but I guess everybody's doing that now.

There's also an option to do this in the settings if it's a Wordpress site.

Kraad said:

I was about to say a 301 redirect but I guess everybody's doing that now.

There's also an option to do this in the settings if it's a Wordpress site.

Click to expand...

Can you speak in more detail?

Both are different
In WordPress go to settings
Simply make www

If you not do redirecting, it will be a duplicate content