Transferring data from one iPhone SIM to another

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iPhone SE 2020 2nd Gen. My current carrier is AT & T . Changing to Spectrum Mobile which requires swapping out AT & T 's SIM card to Spectrum's SIM card.

In doing so how could I ensure that my data (contacts & messages) on AT & T's SIM card gets transferred to Spectrum's SIM card? I'm not sure if my data while using AT & T's SIM card has been backed up to iCloud or not.

How do I see the data in my iPhone that has been backed up to Apple's iCloud? How do I transfer my data stored in iCloud to a new SIM card of another carrier?