Console question about leagues

Hello folks, new to the game and have it on Xbox one. My questions are how do the league's work as far as what you keep/what you lose afterwards, I am in harbinger league. I also cant seem to find an answer to this one next, what happens to the quests/main mission progress and acts I am completing, does the progress start over or something? If so wondering what I should do, I don't want to lose game progression as far as that goes, so wondering my options.

Yes when the league is over everything gets moved to standard. Stash, quests, atlas completion, everything. It might seems like a pain at first, but once you are more familiar with the game, leveling new characters is a piece of cake and becomes the reason to keep playing. The new league gives everyone a fresh start with a new economy and some kind of new mechanic.

When you say moved, does it end? I want to be sure I understand fully, I am under the impression my progress as far as completing the acts is forever gone also...so if I make it to act 10 I have to start over from scratch on act 1 and with a level 1 character?

Neither your characters nor their progress is deleted.

They become Standard (or Hardcore, or whatever) characters.

Thats including atlas progress, uber lab trials, master lvls and hideout decoration as well.
So you wont miss anything - no worries :)