If I switch themes, do I lose all my customizations?

Content like products, collections, pages, and blog posts is stored at the store level, not the theme, so that all stays. What doesn't carry over automatically is theme-specific work: your Theme Editor settings, custom sections, and any code edits, those have to be rebuilt or ported into the new theme.

How painful that is depends on how the customizations were done. Clean sections and snippets port reasonably; tangled inline code edits are slower. Plan a theme switch as a small project, not a one-click swap, and rebuild on the new theme's structure rather than forcing the old code in.