«As a fairish mechanic, an amateur electron pusher, and as a bloke who has herded unlikely junk through the sky, I never worry about theory as long as machinery does what it is supposed to do. I worry when a machine turns and bites me. That’s why I specialize in fail-safes and backups and triple redundancy. I try never to get a machine sore at me. There’s no theory for that but every engineer knows it.»
«The Number of the Beast» by Robert A. Heinlein
What if your MacBook’s SSD failed right now? Would you have a backup ready? Sure, Time Machine is great, but it requires an external drive—one you might not always have plugged in. But what if your MacBook had a built-in backup, always with you?
If your MacBook has an SD-Card slot, it could have. While SD card slots are mostly intended for photographers, there’s an even better use—turning them into always-available backup storage (as usual, no warranty).
Using a form-fitting micro SD adapter
The idea is to use a micro SD card in an adapter custom made for the SD Card slot of that specific notebook. This way, you don’t have the card sticking out of the notebook body, which would be devastating if there is any pressure on it (esp. from above or below). In contrast, the custom-fit adapter ends with the notebook body:

The right side of my MacBook has an SD Card drive, now occupied by the micro-SD adapter (silver/white), a USB-C slot, and a HDMI slot (see image above). The adapter already contains the micro SD card inside and the indentation you see is only so you can remove it later. It fits the MacBook perfectly.
Personally, I used a BaseQi Aluminum MicroSD card adapter for MacBook Pro 14″ and 16″ USHII-420A. It worked well in my MacBook Pro M4. For the micro SD card, I used a SanDisk Extreme PRO 1 TB V30 I card. There are faster options, but this one strikes a good balance between price and performance. I’m not doing video editing and if I did, I would not use a micro SD as storage. The storage is mostly for Obsidian backup (knowledge base) and writing backup (Scrivener files, mostly). While micro SD cards aren’t as fast or durable as SSDs, they work well for incremental backups of documents and notes. Just be sure to choose a high-endurance card.
You’ll likely want to encrypt the card using Disk Utility (GUID Partition Map + APFS encrypted). Otherwise, if someone steals the card, they have your data even if your notebook needs a password (also much harder to notice if someone steals your card than if someone steals your notebook).
Doing the backups
Backups are only useful if you do them regularly. I use rsync in macOS to back up only changed files. I trigger the backup instantly via an Automator app in Spotlight, making it fast and effortless (See my guide here: Syncing external Backup Drives).
The Automator file (saved as an app, with some private info covered):

and the script it executes (again, covered some private information):

Of course, if your MacBook goes up in smoke, the card goes with it. And if it gets stolen, the card will be lost too. But in case the SSD fails, you always have an easily available backup … with you.
Happy backups.
P.S.: Similar solutions likely exist for Windows and Linux, but that’s outside of my expertise.