Everything

All of these things have the charging system in common, and on GM cars and trucks that is a very high-failure system. The generator should be professionally load-tested for maximum current and "ripple". If one of the six internal diodes is defective ripple will be very high and the most current you will get will be exactly one third of the unit's rating. For the common 90 amp generator that means 30 amps, and that is not enough to run the entire electrical system under all conditions. The battery will have to make up the difference. As it does, it's voltage will drop. Computers are very intolerant of low system voltage and will shut down or do weird things. The result is a lot of warning lights turning on.

It is common to go through four to six replacement generators in the life of the car. To reduce the number of repeat failures, replace the battery at the same time unless it is less than about two years old. As they age they lose their ability to dampen and absorb the huge voltage spikes these generators produce. Those are what destroys the generator's internal diodes and voltage regulator and cause the repeat failures.

Auto parts stores that offer free tests usually don't have the same equipment mechanics use. For the type of problem you're having and the symptoms you described you need to have a professional load test performed on the system. If the generator has one bad diode out of the six you will still get one third of the maximum current the unit should be able to produce, and the output voltage will be perfect. I want to see specific numbers from the tests.

Output voltage:...
Full-load maximum output current:...
Ripple voltage:...

If those are good, we can move on and assume you have five different electrical problems that all magically act up at the same time.