That’s a wipe and restore type option. Because the RAM and SSD are part of the SOC chip that also includes the CPU cores, the GPU cores, etc….

So, afterwards you effectively have a new iPhone with upgraded storage. Which you then have to restore somehow.

That is, unless you can find someone who can de-lid the SOC, remove the tiny RAM and/or SDD storage chips, and replace them with new ones. But then you’d have the same problem that the data was on the old chips, and encrypted. And so you can’t just copy that data over to the new chips, you have to decrypt all the data using the key stored on the Secure Enclave and copy it out in unencrypted format, then once the old chips are removed from the SOC and replaced, you then have to copy back all that data onto those new chips and have it re-encrypted from the Secure Enclave.

Anyone who can do that could just give you a new iPhone with more storage.