How do I add custom html files through custom app?

Similar to this post here, I would like to replace certain html files of a certain doctype (accounts/doctype/process_statement_of_accounts.html). I can see in that I can include css or js files but how can I do the same thing for html files?

It doesn’t work this way. There is no per-doctype HTML to edit, it’s all defined in the doctype’s schema. You would benefit from taking [the developer tutorial] (Frappe Framework Tutorial) to better understand how to achieve your desired result.