Today I will talk about Android OS being stupid.

Android lets u as a user move installed apps to external storage = microSD card.

However what is stupid about it is that only a small part of the app will be moved. For example, if you have an app taking 400 MB of storage, then "moving" it will move only about 100 MB of data to microSD card, while the remaining 300 MB of data will remain on the internal storage...

In Android 11 this is still not improved. Imagine having 11 versions of OS over 11 years and never making a crucial feature work properly. Meanwhile app sizes grow ever larger

Disappointing.

I switched to LineageOS recently and discovered it has the option to use the microSD as part of storage.

Now the days of trying to move stuff to SD are finally over for me
I sure don't miss em

Adoptable storage has been part of AOSP since android 6.

This actually makes some sense as certain apps such as widgets (if anyone still uses them) among others can only function from primary storage and for other apps the app data directory is the only thing you can link up from secondary storage. Hence, it will always only move the app data but that can often be quite significant.

However, you can just format the secondary storage device as primary and increase your internal capacity with your sd card. This does have a drawback that your card would essentially lose the "sd card" functionality as it would no longer be removable. That said, I don't think many people remove sd cards from phones these days - I also have it this way. This is essentially what @jedidja said.

This restriction might be in place for security. It might not be cool if everyone with access to sd card had access to app data. With adopted secure it's just as secure as internal flash.

@electrineer That would make sense but when I tried it wasn't the case. You can technically remove the sd card (phone is off) and read it. It will however have a lot of files that are unreadable because part of them is stored on the phone.
It's basically as if you cut your storage into two pieces and tried reading it (and depending on the file system this will yield different results).