Integrating a custom html backend to a wordpress website

If you have to ask, then you have a long road ahead of you.

The best way to integrate code into WordPress is to develop your code as a plugin.

This way all the session management + plumbing is all seamless.

Trying to integrate code with it's own session management is doable + like I said above, you have a long road ahead of you.

https://codex.wordpress.org/Integrating_WordPress_with_Your_Website provides the starting point + you'll save massive amounts of time if you just convert your custom code into a standard WordPress plugin.

Integrate in what way?

If you mean that you have a website at www.address.com that is Wordpress and you want a page at www.address.com/customBackend that's easy to do (assuming you have appropriate levels of control).

If you want to pull Wordpress data onto a normal webpage (most recent post for example), there are PHP functions for that which make that relatively easy.

If you want to import/export data directly between your DB and the Wordpress DB, that's trickier.