Thanks for the reply! I'm glad I'm not the only one confused lol

I found this post on SuperHeroHype forums by a member called djsunyc; I think they explain it rather well and I can get behind this explanation.
Quote:
every time the heroes went to the past - a new timeline occurred.
let's see if i can explain myself lol...
the entire MCU is timeline A. every movie we've seen is in this timeline. thanos got the stones, snapped his finger and 50% died. this is the timeline endgame is based on. avengers kill thanos by chopping off his head. they do the time travel stuff - this is where things get nutty.
timeline A hulk, cap, tony, antman go back to the battle for ny. loki disappearing with the tesseract caused timeline B to happen. to fix this, cap and tony go back to 1970 and by taking the tesseract there, it caused timeline C to happen. hulk getting the time stone casued timeline D to happen. total timelines (4 - A,B,C,D), total new timelines (3 - B,C,D). timeline A hulk, cap, tony, antman return to present timeline A.
timeline A widow and hawkeye go to vormir and get the soul stone. timeline A widow dies. this caused timeline E. total timelines (5 - A,B,C,D,E), total new timelines (4 - B,C,D,E). timeline A hawkeye return to present timeline A.
timeline A rocket + thor go to asgard. they get the ether. timeline F is created. thor brings back timeline F's mjolnir. total timelines (6, A-F), total new timelines (5, B-F). timeline A rocket, thor return to present timeline A.
timeline A nebula and rhodey go to morag. they get the power stone. timeline G is created. timeline A rhodey returns present timeline A. timeline A nebula is stuck in timeline G. timeline G's thanos sends timeline G's nebula back to present timeline A. total timelines (7, A-G). total new timelines (6, B-G).
we are now in present timeline A with timeline A's, cap, tony, hulk, thor, antman, hawkeye, rhodey, rocket and timeline G's nebula.
hulk performs the snap. everyone that died in timeline A from infinity war is back.
timeline G's nebula brings timeline G's thanos, gamora, and his ship (which has timeline A's nebula).
final battle is present timeline A vs. timeline G's thanos + friends.
timeline A wins. all of timeline G's baddies are dusted by tony. (do not what happened to timeline G gamora but timeline A gamora is still dead from infinity war). timeline A thanos was killed in the beginning of the movie and timeline G thanos was killed in the final battle.
the ancient one said to stop the ripple effect of time - they had to return the infinity stones to their original locations in time. at least that's what she thinks.
timeline A's cap goes back in time to put the stones back. we do not know what happens here and what the ramifications are. this is the unknown that the MCU can do many things with.
timeline A's cap is now 100+ years old and returns at the end. imho, he did not come back via the time travel machine b/c he wasn't wearing the suit nor was he in the machine circle. i don't know how he got there but timeline A cap is old.
now all of these new timelines have COPIES of our timeline A people. there's a cap in timeline A (who's now 100 years old), timeline B-G. same for all of our other folks. all we do know is that thanos and nebula from timeline G is dead. thanos, gamora and widow from timeline A are dead. there is a gamora from timeline G somewhere or was also dusted in tony's snap.
i do not know the status of peter parker and high school. the easiest explanation is that everyone snapped did not age in those 5 years and all returned after the timeline A hulk snap. so he's still in high school as is ned. 50% of his high school classmates prior to infinity war - the 50% that survived are all now 5 years older. while the 50% that disappeared are still the same age and are still in high school
i hope that makes sense and it's my best stab at it.