Cannot erase ~500GB from startup disk tldr: how do I erase all of the information from my startup disk? Logging into recovery mode and selecting "erase" has no affect on my disk's "free" space.

While attempting to diagnose an issue I was experiencing, I decided to erase my iMac's startup disk and then restore from a time machine backup. So I log into recovery mode, open up disk utility, and select "erase". Somewhat surprisingly, I see that, while the disk's "used" space drops to almost zero, all of the data seems to be transferred to the "other volumes" category, and the amount of "free" space on the disk doesn't change.

I assumed that this was simply a labeling issue, and that the operating system had earmarked all of this "other volumes" information to be deleted asap. So I opened up time machine and restored from backup. After having completed the time machine restore, I logged into find that all of that "other volumes" data was still taking up hundreds of gigabytes of space, and I had only ~20GB free on the computer.

So I try logging into recovery mode again and selecting "erase." This time it does free up all the "used" space, but it still leaves ~500GB of "other volumes" data.

So how do I actually erase all of the information on my startup disk? Logging into recovery mode and selecting "erase" seems to have no affect. Thanks!!
This will clear all the data on your drive!

You can erase the whole volume by clicking the View button and choosing Show All Devices. Then select the top-level entry and click Erase. Set it to APFS and GUID and click Erase. Name it Macintosh HD (or whatever you prefer if that's not your preference). Then run the installer on your new Macintosh HD and you'll get a clean install with no data.