What do you use for testing?

Title, essentially.

My product gathers raw data every day and turns it into alerts for b2b users. Would love to improve the reliability of it as I add more features, and I understand testing is the way to do this.

Basically, as this thing grows, I want to make sure that the backend doesn't have little bugs that get buried in all the new code I'm writing, and I can deliver a consistent experience for my users.