Offline resources locate in the folder "sitecache". With the option "use offline resources" enabled, the corresponding files in "sitecache/[domain name]/[full path including filename, '?' , '&' , '='. exactly as url]" are loaded first before reaching the network.
PWAs (Progresssive Web Applications) and some web extensions (.crx for chrome & .xpi for firefox) are just packaged websites. Ex. the "kiwix" extension can be extracted to the folder "sitecache/kiwix", then the kiwix extension can be accessed via url "https://kiwix/www/index.html" in the browser. Note: the folder name "kiwix" is used as the domain name in the url.