Does using a case with iPhone cause excess heat to linger around and does it have any significant impact on battery life?

Theoretically yes, but it’s going to be difficult to quantify. Apple warns that there might be heat trapped in a case during charging. However, I’ve never seen an iPhone go into thermal protection from charging in a case. I have seen one in bright sunlight overheating where I had to wait for it to cool down.

Positively false. I wouldn't own such an expensive phone without a good quality protective case. You can also read the earth is flat, but because someone said it was, doesn't make it true.

I’ve used leather Sena folio cases since my iPhone 4. My iPhone 5 and 7 still work just fine with good battery life after years in those cases. My iPhone XS is also in one and the only time it ever feels noticeably warm is right off the Belkin wireless charger, which will make it feel warm with or without a case.

I use the folio cases since Sena makes a good drop protective case and the folio flap means the screen is always covered when in my pocket or laptop bag. I have never noticed a case to make any difference in performance, heat, battery charge nor overall battery longevity.

I saw your post and please let me apologize if you thought I was being condescending. It wasn't my intention. I was just pointing out (perhaps not very funnily) that one can read all sorts of things on the internet, many which are completely false. If you ever want a hoot, google flat earther's. It's amazing, to say the least.