Yup, I saw a video recently on YouTube on a well respected channel claiming to tell the truth about heat in phones and some of it was completely laughable. What was worse were the sycophantic comments saying how good it was, how nice to have it explained and all the while I was thinking to myself that he really didn't have a clue, he certainly wasn't qualified to explain what he was trying to explain.
It is true that under certain circumstances, heat can affect batteries but in real terms, the sort of heat cycles we're going to put the batteries in our smartphones through really isn't going to make any measurable difference to the lifespan whatsoever.
I get up in the morning, my iPhone goes on the MagSafe charger in the office from 09:00 to 17:30, being taken off for use throughout the day. At weekends, it comes off on a Friday evening and then I use it on battery all weekend before charging again on Monday morning. I don't know if it's fast charging, I genuinely don't care. When I'm in the car, it's on an apple MagSafe charger and again, I don't care if it's fast or slow charging, whether it gets warm or not. I know that when the battery is low, it gets warm but as I said before, that's to be expected because the initial charge current is higher.
I've been using rechargeable batteries for well over 45 years and I've heard everything from so called experts. It's at the stage now where I just take it all with a very large pinch of salt.