How to create a plugin

Hi, I am trying to understand if we can create a plugin for Squarespace (like a WordPress plugin) and how / where to get started.

Background: We are currently injecting our JavaScript library in the head of Squarespace sites to enable our product presentation function on pages.
Our plugin function currently looks for specific html on a single root slug e.g. mysite.com/product/
and then, based on the query passed, gets and then loads content (ajax) on to a product page and updates the URL like this:
mysite.com/product/?product_id=......

Problem: However, when done like this, the underlying page 'meta' and 'og' tags all remain the same - so we get no SEO benefits at all as the content is all loaded with JavaScript after the page is seen by a browser (and sharing is a problem too).

Goal: We want to create a more sophisticated general Plugin that would do the same BUT, based on the query string also controls the generation of a page including setting the page headers ('meta' and og tags based on the product) etc - before the page is output.
We have created such a plugin for WordPress - and we have a PHP Class that does this with PHP  - and so we are comfortable with coding etc.

Question: I am trying to find out how to do this with Squarespace.
Can anyone confirm that this can be done? .. and also recommend a specific resource / article etc that explains generally how this can be created for Squarespace - and perhaps a road map for development?

We just want a road to head down - but so far I have been unable to find any resources or guidance on where to start.

thank you!
 

There’s no php or backend access on Squarespace. You can create code snippets for injection (usually in the footer) but you cannot do the same as on WordPress.