You cannot expand the memory of your iPhone.
If you don't see the iCloud+ plan 200GB option, you may have an iCloud+ plan subscription of 200 GB, please check:
Check -> Go to -> Settings -> your name -> iCloud -> if you see 50 GB (or 200 GB or 2 TB or 6 TB or 12 TB) then you have an iCloud+ plan subscription.
If you have an iCloud+ plan subscription, you will receive a monthly bill from Apple, check your email box.
You pay monthly for iCloud+ plan storage.
See also the following Apple articles:
What's the difference between device storage and iCloud storage?
What's the difference between device storage and iCloud storage? - Apple Support
Upgrade to iCloud+ -> Upgrade to iCloud+ - Apple Support
iCloud+ plans and pricing -> iCloud+ plans and pricing - Apple Support
How to use iCloud on iPhone -> How to use iCloud on iPhone – Apple Support (UK)
Manage your iCloud storage -> Manage your iCloud storage - Apple Support