OK, lets try. Many of the features kinda connect together. If you really want deep background on the design, I still recommend the former CTO’s presentation from 2019: A mobile phone that respects your freedom - media.ccc.de Her forum post is probably more to the point about the separation, however…
First I’d like to send you to read some background material on the phones structure and how the schematics and x-rays are available for you to check your own device for tampering and how the hardware kill switches work (former is for general security and latter is connected to the separation. See 4.1 on Frequently Asked Questions · Wiki · Librem5 / Librem 5 Community Wiki · GitLab (and also: Librem 5 component list · Wiki · Librem5 / Librem 5 Community Wiki · GitLab) The announcement of these were in: Breaking Ground – Purism and more about the features in How Librem 5 Solves NSA's Warning About Cellphone Location Data – Purism
Then, I’ll point to a long-ish article in the same wiki about security comparison between Android and L5, which explains a few things that are not straight forward comparable, as a background. See 4.3 on Frequently Asked Questions · Wiki · Librem5 / Librem 5 Community Wiki · GitLab
Here are some quickly found mentions and links regarding firmware-jail (setting limits to card firmware and what they can do): Shipping new SparkLAN Wifi cards with Librem 5 – Purism and Privacy in Depth – Purism
For controlling against what goes in and out of cards, there is also the Open Snich artticle as a bonus (about what’s possible): Snitching on Phones That Snitch On You – Purism
I hope those have the info you seek. I know it’s a bit much and now, after all these years, I see that Purism really should put together a concise whitepaper that - with out any marketing blog wordings - describes the security concept (what the whole system ended up being - positives, weaknesses and to-do list), how L5 hardware and software create this combination/balance of freedom, security and privacy [all relative terms].