Time travel isn’t yet possible, at least in our time. However, if anyone in the future ever figures out how to achieve it the chances are they will have travelled back to our time. Which is mindblowing.
This video looks at all of the different ways time travel works in fiction. And the ways in which authors have tackled the subject have varied wildly.
There’s time travel where your presence changes the future, and there’s time travel where whatever you do in the past has already happened. And that’s just for starters.
Thinking about time travel can really mess with your mind, so my advice is to not think about it too hard. But if you see someone materialize out of thin air, run.