I decided, that, since:

• These web apps are not mine so I cannot change path in their source codes;
• It is too hard to actually manage loading assets and rewriting the resulting HTML in whatever the mainstream server is used (NGINX, Caddy).

- it is easier to switch my subdomain list from domain registar to any DNS hosting (to get more subdomains count) and add subdomain as A record for each web app I need.

That's it, each frontend web app has hostname-webappname.example.tld subdomain. It is the easiest way I discovered myself.