- Look at its device width.
- Work out which media condition in the sizes list is the first one to be true.
- Look at the slot size given to that media query.
- Load the image referenced in the srcset list that has the same size as the slot or, if there isn't one, the first image that is bigger than the chosen slot size.
> ---
Where many people's understanding falls short is the essential importance of the sizes value. In short, get that wrong and the browser is more likely to pick a suboptimal image from the srcset. Put another way, focusing on the scrset - which many seem to do - doesn't help if the sizes value misleads the browser about the nature of the image needed.
I found this artcle some time ago and it changed my understanding for the better. It's not new, but it doesn't need to be.
https://ericportis.com/posts/2014/srcset-sizes/
p.s. I do a lot of WordPress work. It's fine with srcset but usually pretty bad with sizes equal. Even Gutenberg isn't much better. That's right, 35% of the internet is doing this poorly.