How to cache whole website

I have a couple of questions relating to how content is cached.

I find some elements on a page don’t get saved unless I specifically load them whilst online. An example are FooGallery images: the thumbnail version loads as expected, but the Lightbox version does not load unless I had previously opened it whilst online. Another example is audio in Audio Igniter playlists. If I open or play these whilst online then they are saved and will work offline, as expected, but if I have not previously begun playing a track in a playlist it won’t play offline. How can I automatically cache all content from a page, including content that is linked?

In a related question, I would like to automatically create an offline version of my whole website when the user opens the PWA app. Is there a way to list site URLs into the manifest file, telling it to crawl particular URLs (e.g. to cache all content in an /Images folder?)? I’d like to be able to run my website as an app on a number of specific devices, so need to find a solution that doesn’t involve the user clicking every link or image on the website in order to run it offline.

Thanks for any help!