Hi @lubosak, welcome to this forum.
There are no problem for traditionnal devices, because first installation, updates and upgrades follow the same process (via recovery) and use the same file…
Only few phones included Fairphone since the model 3 are using a fastboot install script and an IMG.zip folder for new installation,
i don’t understand why the recovery-compatible file needed for updating or upgrading is simply hiden, specially on the ota server