Can I expand the storage in my iMac?

The best approach is to use an external SSD drive to provide additional storage that you can offload large files to in order to free up space on the internal drive. Fully disassembling an iMac to replace the internal storage is not for the feint of heart, nor recommended by Apple as a user serviceable process.

iMacs have been in continuous production for a quarter of a century and easily over a thousand variants. You have not shared with us your variant, and we can neither see nor touch your computer.

If you want a very answer specific to your iMac variant, please do "About this Mac..." from your Apple menu and tell us what that says for "SUB-MODEL":