How to understand frames px for phones and social media?

Hello, I am wondering why when I click on frames and add a new phone frame, why the dimensions for px are so small at only 428px wide for an iphone 13 pro max, but when I google the pixel dimensions for the iphone 13 pro max it comes to 1170 x 2532 ?

And then when I add a frame for an instragram post it is 1080 x 1080px! This frame for an instagram post wont even fit inside of the iphone 13 pro max frame! I want to understand how to use these together if I was going to mock both the iphone and the instagram posts to be the best quality possible. Should I ignore the default iphone 13 frame sizes figma gives and just create my own frames the size of a real iphone 13 pro max (1170 x 2532)? Should I scale down the quality of the instagram post ? I want my clients to be able to use both the designs I make for phone and for posts in real px values as they would appear on their phone. I’m very confused by this and hoping someone has the answers.