"beta" as in a submissive, passive, feminine, subordinate male - contrast with "alpha", as in the leader, boss, masculine male
"begets soy" - soy is used as a derogatory term because eating large amounts of it raises the amount of estrogen in your hormones. In this case the lack of headstock "begets" or creates or leads to "soy" (or pussy, lame, effeminate)guitar playing
These are just the new-fangled ways of calling someone (or in this case, someones guitar playing) a "bitch," "faggot," "liberal," "pussy." etc
No shame in not knowing what the youngsters are saying, but in this case this is the slang of a couple generations ago (millennials) not the youngsters who would be saying "skibidi rizz ohio 67 toilet aura" or something like that