To free up your phone's memory there are several methods, and the least known is moving apps from internal to external memory. Here's how to do it.
When you buy a smartphone, you almost always have the opportunity to choose between different memory sizes: today you often start with 128 GB, but until last year the norm was 64 GB or even 32 GB. Saving on memory isn't always a good thing, because unless you plan to change your phone often it's easy to run out of storage space after just a year.
In these cases there are three options: move photos and videos elsewhere, delete as many as possible, or buy an external storage memory to move media files or applications into. To move photos these days, the best option is to transfer them to a cloud service like Google One, Amazon Photos, Microsoft OneDrive, or similar. Deleting a few certainly doesn't hurt, but it's a pretty tedious job: you have to look for the really useless, poorly done or duplicate ones and you always risk accidentally deleting some good photos or videos. Moving apps, on the other hand, isn't that difficult and is often enough to free up a few GB of free space and lighten up your smartphone.
How to move apps to external storage
Moving apps from your phone's internal memory to the external one requires, of course, that your phone has external storage. Usually this is a slot where you can insert a microSD card, but very few phones have other types of memory. If there's no external memory slot, then it's useless to go on: there's no way to move apps, you can only move photos and videos to a cloud service.
For phones with external memory, instead, we can go to Settings > App Management > App List and search for the app we want to move. It's not possible to move more than one app at a time from this screen.
Once you enter the tab of the app you want to move, if it's possible to do so we'll find the Memory button that, if pressed, will allow us the Change option. At this point you have two choices: Internal memory (i.e. leave the app where it already is) or SD card (the place where you want to put it).
How to use the external memory for apps
There is another possibility, that is to use the external memory as default space for installing new apps and for all the downloaded files and photos and videos made with the smartphone's camera.
This choice is effective especially with a new smartphone, because if we do this with a smartphone we've been using for a long time we might find ourselves with some files and apps in the internal memory and others in the external one.
To choose the external memory as default storage space we have to go to Settings > Memory > SD Card and from the menu with the three dots we'll have to choose Memory Settings > Format as Internal Memory > Move Contents. Theoretically, all apps and files will be moved, but we can't rule out the possibility that some apps refuse to be moved or don't work after being moved.