Zero your irons/back up sights at your preferred distance (another discussion here ti what’s “best”). Attach your red dot and zero it. Most likely these will be zeroed at the same distance but don’t have to be.
Assuming they are zeroed at the same distance then absolute or 1/3 cowitness are you to you.
If you absolute co witness then you will typically have a somewhat repeatable sight picture. If you do all things textbook everything will line up perfectly. That is not required though and as others have said if your dot is on target you are gtg. Often you may be unable to get in a perfect position so imagine aiming over a wall while standing on one leg. So long as your dot is on target you should be gtg.
Fixed irons compromise the fov when using the red dot but you learn to just ignore them pretty fast and they are always there if you need them.
QD mounted rds can just be removed in the ev nt the rds fails. Taking off and on you would have to confirm the return to zero but quality qd will be within 1/2 moa.
It’s a journey and what works for you and your needs trumps all.
Just when you figure it out you will want to look into 45 degree offset irons or mrds and a LPVO or magnifier for your rds.
Plenty of great info in this thread already and just adding my two cents.
Excellent advice.
Zero the irons and RDS independently. Forget "lining up" everything perfectly. You'll be using one or the other, not ICW each other.
FWIW, I would think about flip irons for the rear set. Otherwise you'll have quite a busy sight picture. The A2 will fade out almost unnoticeable when using the dot, especially if you put on a magnifier