There are different types.

A reversible vasectomy is one where they fill something in the tubes to block the flow.

However even in this case chances of reversal is low and it costs 20X more.

An irreversible vasectomy is where they cut the tube, seal their ends and tie the severed ends together.

It's really difficult and costly to reverse and most of the time impossible. Surgery to reverse a permanent vasectomy may also lead to complications that may or may not be life threatening.

And yeah insurance providers may not cover it.