I believe I found the answer! Startup.cs is not run before the first call to the API is made. After calling a simple Test method in one of my controllers, the BackgroundProcessing method was called That's a bit annoying, as I was hoping I later could create a backgroundworker that loads a lot of data into memory instead of it happening when the first call is made

You need to look at a "queued background service" where you can submit "jobs" to it and it will perform those jobs in a background queue.

The work flow goes like this:

1. Caller sends a request to the service with some parameters
2. Service generates a "job" object and returns an ID immediately via 202 (accepted) response
3. Service places this job in to a queue that is being maintained by a BackgroundService
4. Caller can query the job status and get information about how much has been done and how much is left to go using this job ID
5. Service finishes the job, puts the job in to a "completed" state and goes back to waiting on the queue to produce more jobs

Here is a very long-winded explanation on how it works: https://stackoverflow.com/a/63429262/1204153

Here is an example I made a while back: https://github.com/sonicmouse/ComputationService