Regular files are easy to move. You can get a file manager app to organize them or you can move them over to a folder on a computer and then copy back to the SD card, all via USB cord. You can also set up specific apps, like say your camera, to save its files to the card.
Apps, however, are a whole other story.
First, you need to be using a phone that is running at least Android "Marshmallow" or be able to upgrade to it. Second, you will need the instructions I am including below. And third, you need a phone that has not been restricted by the manufacturer from making this change. Which does happen.
NOTE: If any of the steps are missing on your phone, or they don't work for you, then you have a phone that either has an older Android...