Do I expand or crimp pex water line, and why?
There are different types of PEX and different types of joining systems.

PEX A is joined by slipping a PEX collar around the PEX tubing, then temporarily expanding the end with the collar. Without delay the fitting is inserted and in a minute or two the PEX relaxes and grips the fitting. This type of fitting has a larger internal diameter than the fittings for the other types of PEX.

The other types of PEX are PEX B and PEX C. These latter two are joined without enlargement--a metal band is crimped in some systems and in other systems a metal band is simply pulled into place to secure the PEX tubing to the fitting. Each type has its strong points and weaker points.

One video https://www.youtube.com/watch?v=fhzIRHLOQ_E