In programming, there are server-side and client-side languages. HTML is a client-side language. This means that it can communicate locally with the browser, but not with the server. It is the server that is able to do things like send e-mail. This is where PHP comes in - it is a server side language. It allows your HTML to send data back to the server and trigger an action.
Many of the tutorials you find will reference WordPress. This isn’t because Pinegrow is WP-centric, it is because WP runs on PHP and makes up a large percentage of the internet.
Adding PHP to a page isn’t exactly simple. Another option would be to use a library like https://www.smtpjs.com/. This would probably be an easier solution.