Question about format for backlinks

I have tried searching around but I think I'm searching for the wrong terms, I can't seem to find anything. Apologies if this is a stupid question.

Which of the following am I supposed to use? Does it make any difference?
https://www.domain.com
www.domain.com
domain.com

When checking sites on somewhere like Moz / Ahrefs I see that the page authority sometimes differs when you add or remove the www. in front of the domain.

I just want to make sure I'm not linking in the incorrect format.


Also, I just changed my domain from a .cc domain (after realising a .com would probably give better results)

How should the 301 redirect be set up?
It's currently redirecting domain.com (I used Redirect checker)
But it's not redirecting www.domain.com

Have I done something wrong here?


Any advice appreciated. Thanks!

It's actually redirecting the www.domain.com name but it's not redirecting the https link. From my understanding from a search related to 301 redirecting https, it doesn't really work the standard way, you need to set up some things. Will the link juice be passed on from the old domain if I used to do backlinks with https://www.domain.com ? I will be editing any backlinks