You seem to have the M1 iMac 24", one of the new ones. The storage is not upgradable.
You can use iCloud, Google Drive, Microsoft OneDrive, or Dropbox to offload most of your data from the internal disk. Working on active files and projects is best done locally on the internal storage then you can move it to the cloud. This how most office workers do things. They download files from the server and work on them locally then upload back to the server when done. Frees up space locally. The cloud is nothing but someone else's servers over the Internet. All the cloud providers have enormous data centers and they hold the data for you.
Other options would be some external USB / Thunderbolt4 disks for backups and offloaded storage. But that requires understanding file management and moving stuff around all the time and ensuring you are backed up. If one of those external disks die, there could be data loss.
Another option, especially if you require a large amount of storage and need to expand that storage would be to purchase a NAS (Network Array Storage). NAS uses RAID (Redundant Array of Inexpensive Disks). It's an appliance that plugs into your network with Ethernet and acts as a sort of private cloud server. The most popular brand is Synology and it has a web page to control it. It can act as an Apple Time Capsule allowing for over-the-air Time Machine backups. You can create network shares and multiple people can access the files. You can set security and permissions per user. All point, click and easy to do. A Synology will have at least 2 disks and some have 8 or more. Most household users have either the 2 disk model or a 4 disk model. The cost of a NAS might seem a bit high but when you compare the cost of an ongoing cloud storage subscription to a NAS and the fact the NAS will always be faster on the local network than the Internet... At some point the numbers make sense when you can justify the need.