Hi Olav,
The rear camera lens covers really aren't all that tricky. They're just circles of glass held in with adhesive, so replacing them is basically a matter of picking the broken glass out, removing the adhesive and sticking down a new lens. Really, all you have to worry about is leaving glass chips inside, so clean it out well and don't get fingerprints on the camera lens itself, and of course be careful to not scratch the lens either. I've done it on my Pixel 2, and it's exactly the same for the iPhones.
There's a video guide on YouTube for your phone, but really any guide will work.
[link|https://www.youtube.com/watch?v=YQaEXZd_uSM|How to Replace Camera Lens iPhone 13 Pro Max / 12 Pro Max - YouTube|new_window=true]
The parts are easy to get too; I ran across them with a cursory search on Amazon, eBay and AliExpress all, and they're not expensive. Here's one as an example.
[link|https://www.amazon.com/ASDAWN-Replacement-13-Pre-Installed-Installation/dp/B09PK8NMJL|Amazon.com: ASDAWN Back Rear Camera Lens Glass Replacement for iPhone 13 Pro Max and 13 Pro Camera Glass Lens Replacement with Pre-Installed Adhesive + Repair Tool Kit + Installation Manual : Cell Phones & Accessories|new_window=true]