In my state (Colorado) early voting works exactly like regular voting, just, you know, earlier. Registered voters get their ballots automatically sent to them in the mail. You can return your ballot by mail, drop it off at an official drop box, drop it off at a voting location, or you can show up at one of the early voting locations in your county and vote in person the traditional way if you prefer that. Right now in my county there are six locations where you can do in-person early voting. There will be orders of magnitude more in-person voting locations open on the day of the election, but I think most people choose return their ballots by mail or drop box.

Every voting/counting location is staffed with a bipartisan team of election judges, and election observers. I believe the locations are run by paid county officials, but largely staffed by volunteers who have completed a training program. I've never heard of there being a shortage of volunteers

The voting drop boxes are big reinforced steel boxes which are securely anchored into concrete. You would need some seriously heavy duty cutting tools to get one open without the key. They are placed in front of city offices like City Hall, the Department of Motor Vehicles, or the city library. They're usually in open high traffic areas, and are under 24/7 video surveillance. I believe they're also emptied multiple times per day. I wouldn't say they're impossible to tamper with, but it would be extremely difficult to do so and get away with it. To my knowledge, so far nobody has tried. I'm not actually sure what it would really accomplish. I guess you could destroy ballots, but stuffing one with counterfeit ballots would probably be caught almost immediately.

There's a pretty robust system in place to track who has cast a ballot, how, when, and where. If multiple ballots show up in the name of the same voter, that gets automatically flagged and triggers a fraud investigation. Also there's signature verification system. Every ballot that's returned by mail or drop box must be returned in its security envelope, which has the name of the voter and several unique QR and bar codes containing information tying that envelope to that specific voter. This envelope must be signed by the voter for the ballot to be counted. If the signature on the security envelope doesn't match the signature on file, the ballot gets flagged for investigation, and doesn't get counted until the voter can be contacted to verify it was them casting the ballot and not someone pretending to be them. Voter fraud is really pretty rare here, but it's taken very seriously, and gets seriously investigated. When it does happen it's usually someone trying to cast the ballot of a deceased spouse, or family member, and even that usually gets caught.

There are a lot of safeguards and redundancies in place here that make getting away with voter fraud extremely difficult, but lot of the reason why our system works as well as it does is that people genuinely care about their votes being fairly counted and so are willing to staff and fund the offices who investigate voting irregularities. Our voting system is considered kind of the gold standard for the United States, and I'm lucky to live in a place that has that. Voting systems in other parts of the US are unfortunately not run with the same vigilance or sense of equity.