How do you remove/replace a plumbing fitting that is threaded on both sides when they can’t rotate?
You provide a union, or use fittings that can act as a union.
I'm not sure about those &quot;press fittings&quot;, I've never used them, but PEX crimp fittings will turn without leaking after crimping, so they &quot;act  as a union&quot;- but the default answer for threaded fittings in general is a union, which is specifically designed to be loosened without anything (other than the union nut) needing to turn, so that you can disassemble things that do need to turn next to them.
Flanges are another option - common on heating system circulator pumps, rare in normal household water plumbing.
Third option with copper is to use a solder joint that can be removed non-destructively. I believe (but also don't use them myself) that SharkBites will both rotate, and can also be non-destructively disassembled.