I have RAID, but what happens when it "breaks" and how do I fix it?

I have a QNAP NAS which uses RAID 5 across 3 HDDs. There are lots of good videos online for setting up RAID (even DIY solutions using linux), but how do I know when there's a problem and what's the typical way to maintain it?

I'm guessing that these systems log an indication somewhere that a disk is about to fail, and when I see that log entry I should power down the system and replace the disk with one of the exact same size. Is that right?

Edit: I have found a good overview here: https://yewtu.be/watch?v=OhzrPInWMyo

In essence, he's saying:

1. Enable a recycle bin feature so that accidentally deleted files can be recovered.
2. If the device itself (not the disks) fails, one option is to replace the device and insert the disks in the same order as before. But there's a chance that the disks were corrupted by the failing device, in which case the next point is what you need.
3. EXT4 is a common filesystem format with QNAP and specialised recovery software can help you recover information from the disks. He demonstrates Hetman RAID recovery. You will need to connect all disks simultaneously though, so if you don't have enough ports for this you'll need to get an adaptor of some kind.
4. If just a single disk has failed, then in theory you should just be able to replace it, but there can be complications, which he discusses at 6:37 in the video (the video is just 7:51 duration).
5. RAID is not a backup. Please backup your data too.