I’m trying to get my head round this stuff. I’m sorry if this is a stupid question but how do you boot TWRP if it is not installed?
It’s not a stupid question at all.
You connect the phone to your computer, boot the phone into Fastboot Mode, and then the fastboot command on your computer can boot bootable software images (like the TWRP images for the Fairphone 3) on the phone without installing them.
All that is needed for that and how to do it exactly is described here … https://twrp.me/fairphone/fairphone3.html … just follow the “Installation” section until “Once booted”.
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I have found this very interesting Web page:
“# How to Install OTA Updates in Rooted Android Device” https://www.droidwin.com/install-ota-rooted-android/
Sound very impressive, any thoughts on that ?
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That is how I install OTA updates both on my rooted FP3 and my rooted FP3+. It would be nice if it could be automated! However it’s quite easy to do! But there are some caveats.
It’s also described here:
Note that there is a problem that has arisen since this last page was first posted. I think it only affects Google Pixel phones but the latest versions of Magisk manager have “Install to inactive slot” disabled until the problem is sorted. The FP3 and FP3+ seem unaffected but you can’t use the latest Magisk manager. See also:
I also thought this might be a way to get a FP3 with /e/, TWRP and Magisk.
The first OTA update of /e/ after I had installed TWRP and Magisk on slot B worked fine until the reboot. Then it ended in a screen requiring to lock the bootloader. Then some more attempts to boot and finally the message that only a factory reset would help. I returned to slot B. On slot A I did not have TWRP and Magisk yet. So neither TWRP nor rooting was the problem but the unlocked bootloader. And as far as I understood all the topics I read, the bootloader must not be locked if Magisk is installed.
Did I understand something of the procedure wrong? Did someone accomplish a working OTA update of /e/ with TWRP and Magisk?
Edit: I flashed /e/OS images with fastboot to slot A. Again it does not boot into system mode because “the software can’t be checked for corruption. Please lock the bootloader.”
Then I flashed the latest OTA zip with TWRP to slot A. Same result.
Seems, that the phone denies to boot into system with unlocked bootloader. I can boot into slot B, but not into A. When booting in slot B the splash screen says. “Bootloader is unlocked and system integrity cannot be guaranteed.” So it is only a warning and the phone does not deny to boot into system. I wonder how users managed to update their system with an unlocked bootloader with the descriptions in this and the Fairphone forum. 
Was there something wrong with my installation of TWRP and Magisk? I had installed /e/ with the Easy Installer, so it was installed to both slots. Then I installed TWRP and Magisk, I think this affected only the active slot. Is that correct? So how does the first OTA update work when TWRP and Magisk are only in the active, not the inactive slot?
I need help with this.
I installed /e/ with the Easy Installer, thus /e/ should be on both slots, is that correct? I did not lock the bootloader because I want to install TWRP and Magisk.
Now I installed TWRP, then Magisk, everything works fine. The phone is still in the slot which is active after installing /e/, in this case it is slot B. If I reboot the phone I get a splash screen saying that the bootloader is unlocked, after a pause the phone boots normally into system mode.
I think now I have to change the active slot to install TWRP and Magisk also to slot A. But if I change the active slot to A and reboot the phone I get a different splash screen as described above. It demands to lock the bootloader and eventually ends up in recovery mode. I cannot boot into /e/ in slot A.
What must I do to have /e/, TWRP and Magisk in both slots and to be able to boot into both slots?
Edit: Maybe it is not an issue with unlocked bootloader, TWRP and Magisk but an issue right after installing /e/ :