Can "venit" imply the person was born there?
In (C21st) English, when we say someone "comes from" a place, that's an idiomatic way of saying they were born there or have lived there long-term. It's not really a way of saying they just happen to be arriving from there right now.
So I'm wondering if venit has implications like English "comes from", or if the sentence just means "Marcus is coming / has come from Germany".
True, I doubt that venio had the "origin" implication of the English idiom; it only referred to motion as in "come (back) from or come to(wards) a place or location". To express the former, the Romans would have probably just used an adjective of origin instead: "Marcus ((who) comes) from Germany" = Marcus Germanicus, vel sim.
The difference is usually just for ease in pronunciation: ab before vowels or "h": ab urbe , from the city
ā before consonants: ā vīllā , from the country house
There's a third form, abs : abs tē , from you
I don't think there's anything "wrong" with ab vīllā , but I'm not sure you'd find ā urbe . (Hard to say it: kind of like "a apple" rather than "an apple," versus "a banana.")
It's the same distinction as English a/an.
Marcus a Germania venit.
Marcus ab Italia venit.