Thinking of my house alone, which is about 100 years old.
We have our ordinary plumbing to sinks, toilets, etc.
We have the radiator piping to two of three zones.
All of that has to run safely to the boiler and correctly connect to radiators.
We have the PEX that connects to/is run in a zone with radiant flooring. One end's a boiler, passes through a manifold, then loops through the floor.
We also have sprinkler system, run off separate meter from main water pipe.
Many of our pipes are cast iron, many decades old. Very difficult to work with. Some run through old plaster, some drywall.
Every house is a little different, but in my house alone to do various jobs you'll need a lot of HVAC knowledge that good homeowners aren't going to try some guy with 3 months experience to do on their own. Maybe you'll figure it out -- but no one wants to deal with the mess you create while you gain the experience that brings you to expertise.