The user agent is pretty low hanging fruit, but these days even your most standard captchas / bot detection algorithms are looking at things like mouse movement patterns - a simple bot controlling a mouse might be coded to move the cursor from wherever it is to the destination in the shortest path possible; a human might try for the shortest path, but actually do something that only approximates the most direct path based on their dexterity, where the cursor began, the mouse they’re using, etc.

Tools in this space rely a lot on human use of a computer being much slower, less precise, and more variable than machine use of a computer.