opencart 3.x tutorials

i am looking to do 3 things and am wonderuing if there are any tutorials.
1) either add a custom button to go back to my site/add my custom php menu above the opencart one
2) my site has a user login already how to intergrate that and use it for opencart
3) after a person has successfully completed a transaction, i want to capture what the purchased then use it to update my custom database. (i am selling premade mp3 that will attach to a users account on my site so i store the file name in my custom db

thanks)

either add a custom button to go back to my site/add my custom php menu above the opencart one

To add a custom button, it can be done by simply create a small module and use the layouts afterwards or you could add it directly into your custom theme's TWIG file where the page originally loads at that specific location.

my site has a user login already how to intergrate that and use it for opencart

Vague information. More information is needed; source of subject and its location.

after a person has successfully completed a transaction, i want to capture what the purchased then use it to update my custom database. (i am selling premade mp3 that will attach to a users account on my site so i store the file name in my custom db

I am currently working on a solution on that one, actually, so that users would be able to use their APIs to download files from another store. Have you looked on the Marketplace for such extension that may already be available? It could also be done with PDO if your site hosts on the same environment. Although, while PDO can also be used across different domains, I would not recommend that approach versus the already integrated APIs in Opencart.

re question 2
my site requuires a login before one gets to use opencart.
open cart requires a registered user or guest. i want to be able to have open cart use my db were the users login info is stored to be used as the login for opencart so a double login will not be required. meaning once you log in to my site you are automaticly logged in to opencart to bypass the login/guest screen.

For number 2, this can be done by connecting with the PDO extension with, hopefully, the same environment or with an API.

As for number 3, in order for this to work via cross-domains, a redistribuable license key would need to be provided by a legal provider to redistribute downloads. Opencart allows local downloads from the same domain area. By codes, it can easily be done. Politically, it can be quite complicated since the downloads are no longer linked with any orders but only the products are which those products are already linked to one single domain per store upon purchases as a copy is being sent to the payment provider with the domain name.

number 3 it is all on one domain, my audio files are on the same server