The answer in movies is the same as IRL: it’s to visually indicate that air is moving through the vent. In a movie this can be hard to tell the audience without a possibly annoying sound. In real life you might see this on vents that are difficult to access, probably so an HVAC technician can tell if the vent is working.
Most people who want him to drop out were never going to vote for him anyway.
People (perhaps erroneously) perceive a chance of Biden actually doing it. Trump will never drop out as winning the presidency is his only shot at avoiding incarceration. He’ll probably run again in 2028 if he’s still around by then.
Trump isn’t incumbent, meaning the GOP already had a full primary in the past year. They’ve already determined they don’t have a better alternative. Technically the Dems could have nominated someone other than Biden, but it was never a very likely possibility, so people aren’t as familiar with Democratic alternatives.
Republicans either don’t believe any of the criticisms you made of Trump are true, are okay with them, still believe him to be the lesser of two evils, or some combination of these. This is largely due to right-wing propaganda e.g. Fox News, OAN, Info Wars, etc.
I’m no expert, these are just what I perceive to be happening. There’s probably other factors at play as well.