Two of my three Win11 installs would not generate thumbnails for webm files in File Explorer while using the “Extra large icons” view. The two in question were upgrades from Win10 and the third was a clean install of Win11 (which worked). Most solutions I found on Google were either rebuilding the thumbnail cache or installing Icaros.
I ended up fixing it by exporting the following regkeys for webm from the working Win11 install to the others (YMMV):
Windows Registry Editor Version 5.00