How do I select only the bold text in a long document?

This may or may not work, but first I'd use the styles panel to select all characters with the character style bold. Click & hold on the small triangle to the right of the named style in the styles panel. In the test I did for the screenshot below I copied & pasted text from your post & then applied bold using ⌘ B. Unfortunately, the option to select all bold characters remained inactive until I applied the style to several words. I think will depend on how the bold was applied in the original document.