It depends on the phone you're using and the Android version it's running. Mostly anything that's been released or updated in the past two years will no longer let you install apps in the SD card unless it supports adoptable storage, which not many phones do.
On top of that, only some manufacturers still allow apps to be moved to SD card after they've been installed in the internal memory (if it's possible, you should find the option in the Application Manager on each app's Storage section). And furthermore, it's up to each developer to decide how much of the app CAN be moved, if at all (some allow only external downloads, some only game saves, some only media, some nothing at all). In any case, the app's main file will ALWAYS reside in internal memory, and when an app is updated, since installation cannot happen on SD card, it will be moved back into internal memory for the update and you have to manually move it back to SD (again, whatever portion the dev allows for the app to be moved).