Broken starter, corroded wires, or seized engine?

I have the 4.3 liter engine.

One day I was driving along and I started to notice a very loud intermittent rattling/snapping-ish sound coming from under my engine when one of my friends opened the door to get in my passenger seat wile my car was still running. I decided id take a look at it when I got to the place I was going.
When I got to said place I made the mistake of turning off the car from habit. I got out and popped my hood, then tried to start my car again and it wouldn’t turn over, I’d just hear the short “err” as the starter tried to turn the flywheel but couldn’t make the engine go. Like when your out of charge but I wasn’t, my lights and radio still worked in my car, I had just driven it and started it before fine. I tried jumping it from one of my friend’s cars as he was revving it and it didn’t seem to have any affect on it.

This had happened before a few times in the past month or so, but usually it would fail to turn the engine and id just try again right afterwards and it would start up fine. I banged the starter with a hammer, it also had no effect.
The starter is only a year and a half old. I have had my trans serviced recently, could they have re-mounted my starter wrong and messed it up? I cant remember if the starting problem started before or after my trans was serviced. Electrical connections in that area of my car have fallen pray to corrosion before, my right fog light stopped working and I had to find the rusted area of the wire and scrape it off. I live in humid and wet area and I do drive it in wet conditions a lot. It just rained the night before it failed and it was very hot and humid the next day.
Sometimes when I come to a stop my headlights will get a slight bit dimmer and if I turn the dome light on wile im not moving it sorta flickers. My battery is new too, but I do kill it a lot by being stupid.
I got this car for free so don’t berate me for neglecting to diagnose the starting problem or the voltage problem before something actually stopped working, I don’t put any money into this truck unless its absolutely vital. My dad had a voltage meter he borrowed from work a few weeks ago to install stuff in his expedition but he has since returned it and they gave it away to someone that was leaving the company.

First, I'd check the battery. You may think it is good, but have it checked. This sounds like a classic dead battery.

Second, If the Battery checks good, check the cables and connections. Make sure the wires aren't corroded inside. Usually, you can wiggle the wires, then turn the Key to see if it will start. Also look for shorts where the wire or connector is arcing out on something.

Third, If you're not willing to spend any money on routine maintenance, it is going to cost you a lot more in the long run..

i do all the routine maintanance but wasnt sure if the dimming headlights at stops was normal or not.

today i went back and tried jumping it by putting the jumper wires on the posotive connection on the solonoid and the frame bolt that is connected to the starter ground wire. still just trurns the flywheel a smidge then gives up.

i had it towed to a local gm mechanic. i later found out i could have push started it by rolling it down a hill after it got to 30 mph by slamming it into drive. wasnt too keen on doing that to my poor transmission though.