What is DB recovery mode? How do you shrink DB?

>I am working with a test database and, after a few operations which caused the database to grow very huge, I need to shrink that back to its original state.
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>The first image shows the size used by various files. We can see the biggest one is 5 GB.
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>The second image shows that I cannot make that database diminished under 44 GB.
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>So, I tried various things but I cannot diminish that database. Something is reserving a lot of space.
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>Anyone would have an idea what I could try to really bring that back to its original state?