How to install macOS beta on an APFS Volume. We ran the macOS Mojave beta on our second volume in 2018-2019, so when we were ready to update to the Catalina beta in 2019 all we needed to do was go. Small correction: on an APFS formatted drive, only volumes in the same container share space. Containers on the same disk do not. When you click the partition tab you are given the choice between 'Partition' which adds a fixed size container or 'Add volume' which creates a volume which shares space with the other volumes in that container.

Nix is an extremely powerful package manager that uses a declarative functional programming language to describe dependencies. It stores all packages and build outputs in the nix store located at /nix. This makes it slightly tricky to install on macOS 10.15 (Catalina), because the new read only system volume would prevent creating or writing to /nix. A common workaround is to create a separate writable APFS volume and mount it at /nix.

Before you start

1. As of May 21, 2020, this workaround has been baked directly into the nix installer. However, out of an abundance of caution, it will only work if you don't use FileVault encryption or your Mac is a 2018 or newer model with a T2 chip.
2. If you use this workaround, your nix store will be unencrypted.
3. If you use this workaround, you will not be able to install nix in multi-user mode.

The workaround

Try a normal install

Head over to https://nixos.org/download.html and follow instructions. It might work! Otherwise, if you see the following error, continue with the workaround.

Create a mount point

Create a root-level mount point at /nix using synthetic.conf (run man synthetic.conf in your terminal to find out more)

Reboot if it asks you to reboot.

Create an APFS volume

Install Macos On Apfs Volume

The following commands will create an APFS volume called Nix, mount it at /nix hide it from the Finder sidebar, and configure it to be mounted at boot.

Install Nix

Now you can install nix as normal.

References

This workaround was described and implemented by Daiderd Jordan. I mostly just followed along in the GitHub issue and took notes to help myself and others get up and running on macOS Catalina. The Nix manual has a section on macOS installation with lots of details also.

My mac is using Catalina, I was following the link installing macOS Beta on a separate APFS volume to install

Installing Macos On A Separate Apfs Volume Control

Big Sur Beta. After installation completed, I switch back to Catalina, I found the developer app installed on Catalina won't work, it crashes immediate when I open it.
I tried to delete the volume that installed Big Sur Beta, but it's not working, I am still not able to open developer app. Further more, on Catalina OS Volume, there are two new volume was created:

1. Volume with name update appears on desktop, I have no idea what it is.

2. Volume with name disk1s7 appears on disk untilites, I cannot erase it, I cannot delete it.

The question is, how to completed remove beta OS from separate APFS volume with the volume update and disk1s7?