Question about AR-15 ammo and barrel...

So I'm wanting to get an ar15 either 5.56 or .223. But when I search online, I see articles about how it uses both. How can a rifle use 2 different cartridges with modification? Are there any only 223 or only 5.56 ones available?

Also, I wanted to buy an ar15 with a threaded barrel in case I wanted to buy a suppressor later on... but there's nothing on that either. Do most ar15 have threaded barrels already or do I have to get a tech to replace the system in the gun? And do suppressors install as a part of the gun or are they removable like pistol suppressors? (What's the difference between direct connect and over the barrel)

I've been searching for days and nothing concrete is online. Can anyone with some expertise drop some knowledge about this. Thanks