When loading template parts under the template-parts/ directory, what is the best practice for loading CSS for each part? Should I include the CSS loading directly within each template file? That way, the CSS would be loaded automatically whenever the template is loaded. Is this the recommended approach?
No, search for wp_enqueue
We looked into this at my agency. In almost all cases it is more performant to compile your block CSS in with the main theme CSS so you have a single server call for all of it, instead of multiple separate calls.
Try loading CSS for each template part and register in the theme’s functions file