Beginning with macOS 10.15 Catalina, macOS has changed the drive layout which utilizes multiple APFS volumes so what is shown in the picture is normal:
About the read-only system volume in macOS Catalina or later - Apple Support
With macOS 11.x+, there have been even more changes to the drive layout to increase security:
Signed system volume security in iOS, iPadOS, and macOS - Apple Support
Now, the issue with the free space not increasing after deleting large amounts of data is likely due to macOS and Time Machine (or other backup software) creating APFS snapshots. These snapshots likely still contain the data you just deleted. Normally these APFS snapshots are deleted automatically after a short period of time. As for backup snapshot, the backups must be completely transferred to the external media before they will be deleted so make sure your backups have all successfully transferred to external media.
You can check for APFS snapshots and even thin or delete them by using these articles as guides:
View APFS snapshots in Disk Utility on Mac - Apple Support
https://derflounder.wordpress.com/2018/04/07/reclaiming-drive-space-by-thinning-apple-file-system-snapshot-backups/
Another possibility of free space not increasing is if you made copies of large files on the same internal volume. With the APFS file system when you copy data to the same APFS volume the data is not physically duplicated...only a new link to the existing data is created. A single copy of the data exists on the APFS volume which has two links to it which appear to the user to be two separate copies of the data (this is a space saving technique and also helps to minimize wear on the SSD). When you delete one copy of the data, you are just removing one link to the data....not the data itself. The copied data is only removed once the final link is deleted. I cannot find the link to an article which does a better job explaining this APFS behavior as most articles don't present it in an easy to understand manner and I cannot find the one article which provides a simple explanation including pictures.
