Depends on how you saved things if you sync your accounts to cloud storage you won't really lose much. If you look at your contacts it will tell you where they are saved. https://www.digitaltrends.com/mobile/how-to-backup-your-android-smartphone-to-pc/I am about to change carriers, which will require a new SIM card. What is ON said SIM card that I will lose? Texts? Contacts? Etc.
The answer is that it depends on how you set it up, they can be, or can be stored on the cloud. My contacts are on my online Verizon and Google accounts. Yours may be stored locally. Your messages should be on the phone storage but not on the SIM card, or stored on the carrier servers https://www.androidpolice.com/2019/08/21/backup-sms-text-messages-android/ Txt message have not been saved on the SIM cards in a whileSorry, that didn't really help. Let me repeat the question?
What is stored on the SIM card?
SMS?
Contacts?