I have experience in this (US Midwest)
Most licensing regulation for these things are for professional commerce, and the more general the more they about demonstrating financial means to cover liability and/or a means of taxation, education or demonstration of work is of minimal to no importance. Aquiring general contractor licenses I often found myself using a web 2 fax service to submit pdf documents to a fax number... over T38/VoIP to the building department who could print the fax from their email as they required physical copies to be filed.
It's rarely criminal for homeowners to DIY within building code regulations, permits be damned. When the bank owns part, appraisal or home sales happen, insurance policies are involved non disclosure can be fraud.
My 2¢ on DIY from general contracting experience,
Water:
Undetected water leaks can be fucking expensive, consider location and worst case scenario when making decisions. Don't use shit like AS SEEN ON TV toilet tank gaskets or tool free water valves in a production environment.
Gas:
Thread seal tape is a lubricant that helps seat solid metal (usually) connections to create a seal, not a sealant. (can be welding agent for polymers)
Water, dish soap, spray bottle helps test connections.
Gases have different smells or lack there of, different densities (sink, rise, displace oxygen), can combust in different concentration ranges, etc so understand this.
Electric:
Changing the load or adding potential for increased load on a circuit (new fixtures, receptacles, breaker, fuse, conductor, etc) could cause arcing and lead to fire in unexpected ways. Sections of copper wire get replaced with smaller gauge at some point over a properties history and then get sealed back up in a wall, among all kinds of other wacky shit. Understand the physics and dangers.
Connections are important, and common points of failure. They should be appropriately terminated and enclosed. Use the proper size wire nuts.
All:
Valves, switches, and people are flaky so plan accordingly. Test shut-off or failsafe mechanisms and have a plan for the worst case scenario.