How to get Access Token for background process without any user integractions?

Getting an access token always requires some sort of user interaction initially, either via the "Generate" button on the app's page on the App Console (when just getting an access token for the app owner's account), or via the OAuth app authorization flow (which can be used for any arbitrary user, not just the app owner).

In either case, once the access token is retrieved, the app can store and re-use it without further manual user interaction.

Long-lived access tokens don't expire, so they can be stored and re-used indefinitely (unless/until revoked). Short-lived access tokens expire after four hours though.

With the new system of short-lived access tokens and optional refresh tokens, in order to maintain long-term access without further manual user interaction, the app would need to request "offline" access, to get and store a refresh token, in addition to the short-lived access token. A refresh token allows the app to retrieve new short-lived access tokens on demand, whenever needed, without further manual user interaction. The "Generate" button does not currently offer the ability to retrieve refresh tokens, so you'd need to use and implement the OAuth flow in order to get that. You can find information on how that works in the OAuth Guide and authorization documentation. This blog post may also be helpful if you haven't already seen it.