So it's not uncommon to have some of city mayor, city council, regional, state and federal and European elections fall onto the same date. I think the "worst" I've seen as a volunteer was 4 elections on the same day.
Counting process starts by splitting the ballots (if we have all of them in one urn), and then counting and tallying them one election at a time. The ballots are distinguished by different colored paper.
One voting district is usually around ~1000 voters, out of those, before Covid ~10-20% did mail-in-voting, during Covid it was ~40-50%. Then with a normal voter turnup of around 60-70% depending on the elections that are held, we end up with a few hundred ballots to count per election, with a staff of normally 8 people... Polling stations close at 6pm, and the latest I have left the voting station after counting was 8:30pm.