Every now and then I need to find a file I deleted some time ago, or which was an older version. The system just chugs away in the background without my attention. The two SSDs are formatted as APFS with the external Samsung X5 connected via Thunderbolt. The two WD drives are formatted as Mac OS Extended (Journaled) and connected via USB.

None of the drives have more than a single partition, except, of course the internal SSD, which MacOS formats as 2 partitions: Macintosh HD and Macintosh HD - Data. So TM backs up what's on: the 2 TB Studio internal SSD, and a 500 GB Samsung Portable SSD X5, I use for for Media. I exclude the two WD Elements drives in TM so they don't back themselves up, but perhaps that's built in now, not sure. I have TM set to use dual mode, i.e., it alternates between the two disks A then B, then A then B etc. I have TM backing up to 2 WD Elements 8TB hard disk drives. However, I also use an offsite backup process (CrashPlan) as extra insurance. At least that's the way it was many years ago. I don't back up anything to the Cloud because that process doesn't back up everything. I backup everyone's iOS devices to their own MacOS machine. I've used it to setup new machines using Migration Assistant, and used it for three family members and a friend. I've used TM since it was introduced and found it very reliable. I recently transitioned from a 2017 iMac 27' to Studio.

Backup external hard drive time machine