Non-airline civil service job here. You can get hired off the street, go through all training, and pop out the other end and head back to your job. Actually, even more, the ~2 years of UPT pipeline actually don’t count toward the 5 year USERRA limit.
When you spit out the other end, the above posters are right about them wanting to make sure you can feed your fam/pay your bills, which is why they ask. There is the potential of just picking up orders (Troughing/Bumming), but COVID might throw a wrench in the availability of that as folks come back from the airlines. Guard seems to be more available for orders, but that’s just anecdotal from what I’ve heard and it can vary greatly by unit/airframe.
The Reserves offer TDART, which is an up to 4-year, non-orders, full-time GS job. You’re in your flight suitm working at the unit, but it’s not on orders, so you don’t get any USERRA protections if you’re trying to hold onto your old job and you might have to pick between the two. But, the positive is that it’s an “entitlement,” which means that you’re owed the job and it’s yours, if you want.