If I download an upload a theme – does that make it act like a child theme?

So I’m struggling a bit with understanding what exactly makes a theme a child theme. I understand how to create one – but it is too late for this project I am working on. I downloaded a third party theme “Tesseract” by tyler moore. It is not attached to any theme in the wordpress store. Can it still be updated and I lose all of my customizations?

What is the best course of action for me to take here? Should I just make a child theme that references this as a parent theme? My customized css is in a separate plugin, so i don’t think it is in danger(I also have physical copies of it).