So when a new league starts, all your characters from the old one goes into standard and you have to totally start over with nothing right? What else changes from league to league? how the hell are people are talking about endgame when if you have to start over you have to level your character back up again? 

When you start the league, you start completely fresh, along with everyone else.

Every league comes with a big patch that usually has balance changes or what GGG refers to as an "expansion" which is something like war for the atlas that changed up the mapping system drastically or like fall of oriath which added new acts to the game. Changes to skills like what's expected with incursion, new skills, ascendancy rebalances like came with bestiary. Other things to kind of refresh the meta a little usually. Unless you're super hardcore about the game, it can get really boring when the same skills and ascendancies are so good, and if your goal is exp or currency then you might just ignore the league most of the time, in which case a lot of times you need something else to keep you engaged.

People are talking about endgame because for players who have played a ton or practiced a lot, you can finish act 10 in like 4-6 hours pretty easily. Check out DeadAndDoom, he's the best racer there is (racing is basically starting from a certain point and getting to a certain goal as fast as you can, it can be exp in a certain time or progressing to a certain point.) And other streamers and such have done videos on league starts, engineering eternity probably did, I know raizqt has some videos of league starts or race ideas, zizaran might too.

I'm not as good as they are, and it probably takes me 10-15 hours to get to maps, also partly because I don't like playing the way that gets you there faster (maximizing impact of exp by being underleveled, ignoring a lot of mobs and just running to boss and killing big packs or blue mobs) I just run to where I want to go and kill along the way however I think is fun. Do you want to poe currency purchase? If you do, you can visit U4GM to have more guides and methods about this.