What a clean fixture swap looks like
Most fixture replacements are honest, contained jobs. A kitchen faucet on a standard three-hole sink with working stops underneath is off, out, in, on and tested inside an hour. A toilet on a sound flange is not much longer.
We isolate at the fixture stops rather than the whole house wherever the stops hold, pull the old unit, clean the mounting surface, and set the new fixture with fresh seals and braided stainless connectors. The thin plastic supply lines that ship in a lot of faucet boxes stay in the box. They are the cheapest part of the assembly and the likeliest to end up on an insurance claim.
Then it gets run. Hot and cold, full flow and trickle, drain checked under load, a dry paper towel around every joint underneath. Silicone goes on last, and we leave the back of a toilet base unsealed on purpose, so a leak shows on the floor instead of soaking quietly into the subfloor.