Watch Da Birdie
I buy cakes for myself on my birthday it's not weird lots of people do it I bet
There's never been an "official" explanation, though seems like anytime it is brought up, it's only mentioned as being the far future of just UC.
If you did want to fanwank them all into one timeline, I think UC > X > Wing > G works best.
I don't know, that seems to imply that the world has reset multiple times already since in Wing they specifically state that the Tallgeese was the first mobile suit. And then you have the fact that each reset resulted in history moving the exact same way up and having the same countries formed.
And after three flawed attempts at creating peaceful space colonies, they achieved success through turning Earth into an MMA Ring and creating a colony shaped like a sombrero.