The first hires I made early in my career, I had no idea what I was doing. I interviewed people and tried to get a feeling for whether they would be a good hire. Some of them ended up being great hires and some were terrible. Bad hires take up a disproportionate amount of your time, create stress (and sometimes make legal threats), and sap productively. Nonetheless, overall, I guess I was lucky enough.
For a hiring manager, deciding who to hire when filling open positions is among the most important decisions you make. Good hires usually mean high productivity and low stress. Over time, the quality of a manager's hires is likely the difference between getting promoted and getting fired. Learning to hire well and using a process that scales is a valuable skill that will make a huge difference. Don't rely on luck!
As I progressed in my career, at some point I realized I needed a system. I couldn't do it all myself and I needed something that let me know whether we were succeeding. I worked with my team to create criteria for the roles we were filling. We came up with ways to evaluate candidates. The system was probably pretty bad at first. But we kept looking at the quality of our decisions and tweaking the system trying to make it better.
Without a reliable process, personal biases and luck become a big part of the decision. Establishing a process, following the process, and backtesting the results leads to better hires and better performance as a manager.