How much free space do I have?

Free and available are not the same. Free hasn’t been allocated. Available is ready to use. An SSD cannot recover used space immediately. It could take overnight to reset the storage to make it writeable.

You have a volume group selected in Disk Utility. That group contains several volumes not shown. The Data volume (which you see in the Finder window) is one of them.

Steam probably has no idea.

To add to what @Barney-15E has mentioned, "Available" used to be called "Purgeable" which I think is a much better & descriptive term...not sure why Apple changed it to the more confusing term "Available". At some point macOS will release the storage shows as "Available", but who knows when that may be, but you will only be getting about another 130GB of free storage when that happens assuming the system doesn't need to use more storage for normal operations. Maybe try rebooting the computer, but more than likely it will be released at some future date.

FYI, you are getting very low on free space. You really don't want to lose much more space since you always want to have at least 20GB free at all times for the normal operation of macOS. The more free space the better since 20GB can get used up very quickly with temp & cache files for macOS and your apps. 40GB is a much safer area. When the SSD has very little free space left, the speed of the SSD will be much slower since the SSD must do a lot more work and it may cause the SSD to wear a bit faster.