Different organizations define residency differently. There is no single accepted way to prove residency. Sometimes a drivers license does the trick. Usually to get a a drivers license you need a utility bill or a lease. To get in-state tuition you need to prove a year of residency. Proof of residency for state taxes is more complicated, and varies by state. Residency is different than domicile, etc. As someone who has moved around a lot I've bumped into quite a bit of this.
There's nothing in the constitution saying that a person needs to own or rent a house to vote.
They do, according to the statue I literally just linked. There is no legal requirement for a fixed address. If you disagree, you're welcome to cite the relevant text from the ga code.
O.C.G.A. 21-2-217 section 15, specifically, gives county registrars broad authority to accept people based on exigent circumstances, including homelessness.
Which people without fixed address do not. We are talking vagabond voting without residence so they used the shelter's address.
You can criticize Veritas all you want but that is not the focus here. Someone was caught on the record admitting to committing a felony in the state of Georgia.