Third Party

Will somebody explain this to me in a way that makes sense? I keep hearing that a third party vote goes to one or the other leading candidates. How does this work exactly? When people vote third party, does someone then dump those votes into Trump’s box or Biden’s box?

Also, Democrats claim that a third party vote is a Republican vote, while Republicans claim a third party vote is a Democrat vote. So, which is it?

I’m not voting third party, but I don’t like how people from the two leading parties shit on people who want to vote third party.