Why did some troops during WW2 have semi automatic rifles, and some had submachine guns?

by SimCD

I.e, the m1 garand, m1a1, Thompson and m3 "Grease gun." Did they undergo different training, like I assume machine gun troops did, or was it completely random?

Did any of these troops carry handguns with them?

Georgy_K_Zhukov

We've had questions on similar matters before, so check out this one by /u/the_howling_cow, but also this old thread, this one, here.