This might surprise many, but unit tests weren’t common a couple of decades ago, even if professional code bases. You just had to design your architecture so it was resilient, structure your code so it had as few secondary effects as possible, and manually test throughly. It was frowned upon to test superficially or “moving fast and breaking things”.

I know, boring, but the end result usually was rock solid, there were no “we’ll patch it later” opportunities.