Connecting customers through our web app

Hello, everyone:

Let me explain the goal of our app, so you can tell me the right approach.
In our team, we have Jira Cloud with our users to get track of pending issues. We have the “helpdesk portal” where our customers ask for new issues.

Now, we are making a web-app to integrate all information we have for our customers in our database and Jira, so I need to connect to the REST API to get that info. The problem is that our customers are the one that need to connect to Jira.

I implemented OAUTH2 to connect to Jira with my user, for testing purposses, but I have two major problems I don’t know how to solve:
1 - Our customers don’t have a Jira user per se, just a customer account, and I’m not sure if they must login in Jira like a regular user. I mean, I know they can log in because I did with a test account, but to use the REST API…
2 - Is there a way to connect via OAUTH2, using the “Jira customer account”, without leaving our website? In the documentation
https://developer.atlassian.com/cloud/jira/service-desk/oauth-2-authorization-code-grants-3lo-for-apps/
It says that I must open https://auth.atlassian.com/authorize , log in, and wait it to return to the provided “callback url”. Can I make this login “unattended”, without using “google login” or anything similar?

I can’t use Basic Auth, because it’s not reccomended (and it’s about to dissapear), and I want to use REST API with our C# NET Core app. I don’t see any example, and I had to do a lot of trial and error to make OAUTH2 to work.
Also, we’ll need to make another app that will connect to Jira every night to update the new info uploaded to it, and it can’t ask for manual log in.

Thank you.