Apps Script is basically the equivalent of VBA for Excel. It allows you to connect your Google Sheets with Gmail, Google Docs, Google Drive, Google Calendar, and numerous other services. A TON of scripts can be found online, and even with only a rudimentary knowledge of coding, I have been able to Frankenstein scripts together to accomplish numerous things.
The usage limits are very generous, and it will not end up costing you extra $$ to create very sophisticated automated workflows. If there is specific data you need to get into a Google Sheet from an outside source, I've found most out-of-the-box tools like Zapier to be very limited, whereas Apps Script had the power and customizability to do exactly what I needed. Review collected by and hosted on G2.com.
Apps Script uses some version of JavaScript, and there is definitely a learning curve to get started with it. Getting reliable scripts does take some skill. The time investments have paid off for me, but there's definitely a lot of trial & error, YouTube videos, and StackOverflow searches required to get from Point A to Point B if you're a novice coder. Review collected by and hosted on G2.com.

