Not quite sure what the issue is....

I have a 2000 Monte Carlo as, and it’s been
having some issues starting.

Here lately, it’s been intermittently not starting.
Initially when I thought it was a bad battery, I
tried having a friend jump me, and we stayed
there for a good half and hour with the cars
connected; it wouldn’t respond to a charge,
so I left it at their house (they let me borrow a
car so I could go to work in the morning). I
came back that night, and the first thing I did
was start the car, before doing anything to it -
it started up beautifully.

Several weeks later, I had some work done on
my car, and it started having the same sort of
problem again -it would act like it was a dead
battery, but when I got a jump, it wouldn’t
respond, but letting it sit for a day, and trying
again, it would start up like there’s no issue.
A few days after THAT incident, I go to get
gas, and when I turn off the car, I hear a
“click” from the speaker, and I noticed the
light in the USB port I have in the cigarette
lighter was not on like it usually is, so I tried to
start it again and it wouldn’t even try to turn
over.
I called my wife to try and give me a jump,
and in that time I decided to pop the hood
and connect the jumper cables. When I did, I
noticed that my USB light had turned on, so I
decided to start the car, and it started up...
with me just connecting the jumper cables to
my battery and nothing else.
I went ahead and went to work, and in the
time I was driving to work after that weird
restart, it was running fine. When I got into the
parking lot at work and turned of the car, I
heard that pop again, and it wouldn’t start.
After work I had a co-worker try and give me a
jump. It wouldn’t do anything. A bystander
offered their assistance, but it didn’t turn over
then, and area security tried a battery charger,
but still did not turn over.
I had to call my dad to pick me up and bring
me to my house.

I just don’t understand what is wrong with the
car.