I'm unable to free up space after factory resetting my MacBook?

I got my sister's macbook and decided to factory reset it by following the instructions I saw online that instructed to erase the Macintosh HD. After that, I only had 20GB of available space and after reinstalling the OS, it was reduced to 8GB. I checked the storage and 100GB of it is used up by "Other Volumes in Container".I can't update the software now since it needs 16GB of space. I don't really know what to do since I'm afraid of messing it up. What do I do with the other volumes in container?

[Re-Titled by Moderator]