Follow along with the video below to see how to install our site as a web app on your home screen.
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https://www.gsmarena.com/samsung_galaxy_j7-7185.php
So wiping the system cache partition will have no affect in any way to those 'storage full' messages. Your phone's internal storage is divided into several partitions, most are dedicated solely to the Android operating system and one, a user data partition, that's just for the user (you). Those system partitions are isolated from your access to them, you have complete access to everything residing in that user data partition, you only have very little to no access to the system partitions.
So all that said, when you look at the Settings >> Storage menu in your phone that should give you a good, cursory view of what files and things are still taking up space in your internal storage. And again, ignore all data tied to the operating system. They're not readily accessible to you so no need to fixate on them. Are there apps you've previously installed but don't use any longer? You mentioned that you store your photos on your microSD card but what about things like videos and music files?