Hi,
The problem with filling up the space might be due to the phone-cache. E.g. when you update an app, the apk-file of the new version remains in the cache after the update is completed. You can manually delete those files using a file manager or use an app to clean the cache.
Regarding the microSD card: the card can be used as internal storage or external. I would highly recommend to not use it as internal storage.
Take a look at this wiki:
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