I don't get it

Advice before reading: eanglish aint my first lenguage

Hi! I'm kind of "new" to mech touch (3,000 chapters) and have some doubts about the worldbulding and some lore pices, and I'm loking for someone who is further ahead to explain them, plis :)

1-In some cases it says that the Therans are one of the top 2 civilization thanks to their location in the center of the galaxy, but my problem is that in some cases it also says that they have for homeground the old Earth (our Earth), but we are located in a branch of the galaxy, so the question is, How is that posible? Did they move Earth to the center whit some especial gun or what?

2-I don't really understand good enough the order or star cluster, star system and the likes, to my understanding, star system is like the soler system, but after that I have no idea, so, Could someone please do me a list of orden and explain each one? If it's not a big problem, obviusly

3-Do you recommend me to be reading the Reddit? Cuz I took a peak and it seemd like most people talk about recent events, and I don't really want to get spoilers

Thanks in advance :D