Pex is modern flexible plastic type of tubing for water lines.
Very popular, replacing copper in much new construction.
It comes in rolls, or ten foot or longer straight pieces.
It can be joined to copper with special barbed fittings that attach to the pex, with a ring that clamps around the pex with a special tool that you can rent.
There are at least two types of connecting rings.
One gets squeezed all around to tighten.
Another has a bump on the side that gets squeezed, which pulls the whole ring tighter.
There are also Sharkbite brand fittings that can attach copper tubing directly to pex, with no soldering or special tools.
Sharkbite fittings seal with an o ring, and they grab on with metal teeth.
They can be removed with a small cheap tool you can buy.
Home Depot and many places sell Sharkbite fittings.
You can search “copper to pex Sharkbite fitting” on Google.
It is approved to be hidden behind walls and framing, very good track record when properly done.