How does SD "Permanent Storage" works on Macs?

Hello,

It's my first time asking on this site so I will be direct,

I've been having questions about that flash drive people were talking about back in 2013-14, so I don't really know how SD cards are capable of increasing storage in Macs and really would like to know if that's possible to do the same with external HDDs/SSDs without installing OS on them?

*I use some HDDs with MacOS installed in order to work with some specific apps, which wouldn't work on my working machine (due lack of memory or hardware limitations), but if increasing storage would be possible, then installing OS over and over again on different drives won't be necessary.

Thanks))