I sometimes see SIM cards talked about a lot in phone privacy discussions. An example is a part of an answer to another question I asked about the limitations of GrapheneOS:
You can buy a non-KYC’d SIM as long as you recognise that still has significant limitations. As soon as you use it at home / at work it’s trivial to link to you, still marginally better than a KYC’d SIM though.
I’m having a hard time finding anything that provides a general understanding of SIM cards, how they work, and how to approach them in terms of digital privacy, which I feel would help best process information like the above. The PrivacyGuides main page has nothing on them. Can someone provide an explanation below or a link to a site that explains them?
As far as I know, there’s not much you can do with or to SIM cards to enhance your privacy or security but just making sure you don’t store any contacts on it.
Also, I don’t know where you’ve seen SIM cards being talked a lot about and find it hard to believe if they are talking a lot about, this info you’re asking may not also be accompanied with said discussion. But anyways, I just find it odd.
As @micdan mentioned you can store contacts in contact storage on your device, or you can store them on apps (e.g. email providers like Proton / Microsoft Outlook, etc.)
If you are using stock android, then probably you are already syncing your contact to your Google Account.
With the Simcard there is not much to do beyond avoiding its use, although this is not an option for most people. I keep contacts inside Fossify’s Contacts app - you can also do this with Contact You - which prevents them from being seen by other apps even if they have the contacts permission. The Fossify application only saves the contacts locally, but you can export them and save them on your computer or application like Proton.