Use time machine with sd card

Thank you. I will buy a 4 TB passport or something similar

Something to consider:

You can google "3-2-1 Backup Strategy" for more information.

Basically three copies of your data, two different methods, and one offsite.

I would submit you are better off with more than one monster size drive...

Boot clone https://discussions.apple.com/docs/DOC-10081

How to use Time Machine to back up or restore your Mac: https://support.apple.com/en-us/HT201250

Use DiskUtility Restore feature  Restore a disk using Disk Utility on Mac - Apple Support

Can I use time machine on my MacBook Air to back up to an sd card in the MacBooks sd slot

Hardly seems a practical arrangement.

Backup disks you can use with Time Machine - Apple Support

It is not like it is not supported... it might work, provided that:

1) The SD card is formatted properly for this (Mac OS Extended format, with a GUID Partition Table)

2) The SD is big enough

Of course, 2) is a big obstacle, since in most cases a large enough SD card may not even exist.

(and I am not even talking about the recommended size, which is several times the size of your system drive; even the same size may not be possible)

Even you can buy an SD card large enough, you do not want this. SD cards are way too slow.

And it would not be cheap. You'd be better served even by a slow 5400rpm 1TB hard drive for maybe $70 than with a $200 256GB SD card...

A Time Machine drive should be 5 times larger than the drive that you're backing up. So, no, it doesn't sound feasible.

Additionally, SD cards are really not of a quality for permanent storage of important data.