They can:

• stay home - this results in whomever gets the most votes from everyone else getting the electoral votes for that district/state. In many districts, this benefits trump
• write in someone else - more clearly shows protest, but what that actually does or if anybody really counts it, I have no idea. Effectively the same as option 1 for the outcome of this particular election
• vote for a third party - basically the same as the above, though certain things do happen if a party gets some percentage of the vote, but not until the next election. The outcome for this election is the same
• vote for one of the two major candidates - self-explanatory

How much not voting or protest voting impacts the actual results kinda depends upon the district and state. However, even in a seemingly secure district or state, enough people protest-voting could actually have a negative impact in that particular election (though I find that fairly unlikely). I vote in a rural district that supports trump. Since he’s objectively worse in basically every way and has indicated that he’s willing to let Netanyahu’s government do whatever they want, I feel it unethical to do anything but vote against trump which, given what I wrote above, basically leaves Harris. If I know that trump will be worse, and I know that doing anything other than voting for Harris in my district helps cement trump, then I must vote for Harris or I’m just helping trump.

We are here because nearly everywhere uses first-past-the-post and voter turnout isn’t great, particularly by voters in local and regional elections. I vote as progressively as I can in local elections and advocate for ranked choice or rated voting wherever possible. People in power tend to hate ranked choice voting or similar because it removes what almost always just becomes a two-party fight which often leads to tactical voting rather than properly representing people. The state in which I vote also has a huge gerrymandering problem, but that’s another story.