Yes, excellent idea, it's easier to fit, more lasting and often cheaper.
Because you will then get a 'galvanic reaction'.
Yes, plastic pipe, PEX, can replace copper anywhere now.
unscrew it replace it, copper pipe coil easy to replace. Two pipe nuts about 3/8"
Detect the leak and replace that part of the pipe. It is usually better to replace the whole piece of pipe. By removing the junk plastic and replace it with quality piping such as copper DWV or NHCI
Galvanized pipe will be silver/gray color. Copper will be copper. Use a magnet, it'll stick to galvanized pipe but not to copper. Drinking water lines should not be black steel pipe.
For copper pipe, lead-free solder is recommended to ensure safe and effective connections.
That woulld depend on the piping materials and local codes as copper TYPE L , K or M should not be threaded and plastic piping should NEVER be lead wiped to a brass ferrule and Cast Iron should not be glued
When I have to connect copper to lead "D" I usually solder the copper to the lead or wipe a lead joint depending on the diameter To connect copper to cast iron ,.... If there is a hub I caulk the copper directly into the CI with oakum and poured lead (molten) OR one can solder / braze a female adapter fitting and screw the copper into a CI female fitting
To efficiently replace a copper pipe with PEX in your plumbing system, follow these steps: Turn off the water supply to the area where you will be working. Cut the copper pipe using a pipe cutter. Clean the ends of the copper pipe and PEX tubing. Install the appropriate fittings to connect the PEX tubing to the existing plumbing. Use a crimping tool to secure the connections. Turn the water supply back on and check for leaks. Insulate the PEX tubing if needed.
Replace them with copper or plastic pipe.
1/2inch
If you had been advised why ask again?