Symptoms:
When translating the 'header' template part, there are no strings found. Additionally, my block theme has only 1 header template part, but there are 3 listed under the 'Template Part' section in 'Translation Management'.
Questions:
What am I missing when translating the 'header' template part since no strings are found?
Why are there 3 header template parts listed under 'Template Part' in 'Translation Management' when my block theme has only 1?
Solution:
Under "Translation Management" is a section "Navigation Menus". For some reason I thought this was for classic themes and ignored it. Turns out that this is very Navigation Block menus live. The name "Main Menu" is set per default in the Block Editor Inspector Sidebar under "Advanced" (when editing the Navigation Block), which was the reason why I got confused.
