From what I understand...

and I'm not citing this, so keep your skeptical spectacles on...

They don't read the content of the message, but they take the metadata of it and cross reference it against their other platforms. This means they know who you're talking to (phone numbers) and build contact networks this way. if they can get a positive ID to Facebook or Instagram accounts, they can attach lots of other wizardry to it. They know who is talking, how frequently, and at what times.

Signal and telegram don't have those sorts of associations.