Tip for non-native speakers:

Say "that" more often than you say "this". "That" is more specific. It indicates an awareness of something, or the desire to be informed about it. You know what you have in mind, or you're TRYING to know what you have in mind.

If you see something far away and don't know what it is, say, "What's that?" Or if someone says a long sentence to you and you want to know what a particular word means, say, "What's that?"

When you say, "What's this", we tend to imagine you holding whatever you're talking about or pointing to it when it's close enough to touch. And it sounds like you don't know much about it.

Another thing that Americans say (idk about Brits) is "what is this" when we don't know what's going on, and aren't feeling particularly polite about it. Again, it's non-specific. Like, "What are you people doing here?" In this case, we say "this" because apparently something is going on, despite us not knowing what it is.

I know it's a bit confusing but remember,
say "that" if you're being specific; say "this" if you're being vague.