In my setup I still use reverse proxy even though all of my services are inside a VPN. IMO it is just more convenient to have services accesible as subdomains or subdirectory than as different ports.

But pretty much in the same way as the YouTube’s frontend requesting content from YouTube’s backend. This is an equivalent of you loading a video on YouTube then going to developer tools and copying links from the Network tab. AFAIK all tools (Invidious, Piped, yt-dl) work this way.