How do I record a Google Meet, and can I record if I'm not the one who started it?

Recording needs a paid Google Workspace plan (Business Standard and up), it's not on free personal Gmail accounts. If you have it: in the meeting, click Activities (or the three-dot menu) > Recording > Start recording. You need to be the host or in the same organization as the host, guests can't record. Everyone is notified when it starts. When it ends, the recording saves to the host's Google Drive (a 'Meet Recordings' folder) and a link is emailed to the host and organizer.

The paid-Workspace requirement is the catch. On a free @gmail.com account there's no record button at all, you'd need a screen recorder instead.

It lands in the organizer's Google Drive, not yours, even if you started the recording. So if you're not the organizer, ask them to share it afterward.