I used memories for a while but Immich is much better.
I use an external library because I export images from Lightroom Classic and that's where I throw them in YYYY/MM directories.
I could import them directly into Immich but I had problems with the Lightroom plugin I used. Especially when exporting hundreds of images at once.
Any chance you’re doing this for film photography? I also use a plugin (Negative Lab Pro) for negative inversion of film scans that keeps me stuck on Lightroom Classic. It would be great to get a pipeline beyond Classic but with the ability to jump back and re-edit. Curious if you have more details on what you do/don’t connect into Immich from Lightroom.
It is not unusual for there to be hosting or intermediate storage of images and other files, and from the phone you may tap a link or something to download/access that file, instead of having it automatically download and appear immediately, due to bandwidth and resource constraints.
In France, I'm "charged" for MMS, too. But that's actually considered "data", so it's deducted from the "internet" envelope which is quite generous (at least for my needs: I have multiple dozens of GB for under 10 € a month, of which I only ever went above 10 when backing up photos during a vacation with no wifi).
I haven't looked at it but I bet it's written in handwritten ASM. You can disassemble that. It probably won't be easy but with a memory map of the C64 and 1541 you could tell how it's working and learn from it.