How to make remove free space and make APFS container take it up
This answer was tested using Big Sur (macOS 11.0.1).

Boot to Big Sur.

Close all applications. Open the Terminal application.

You can create a new APFS container.  An example is given below. Assume the output from diskutil list is as follows.

The command to enter would be as follows.
diskutil addpartition disk0s1 apfs NewAPFS 0


Clone (replicate) your existing Big Sur installation to the new APFS container. An example is given below. Assume the output from diskutil list is as follows.

The command to enter would be as follows.
sudo asr --source /dev/disk1s5s1 --target /dev/disk2 --erase


An alternative to the asr command would be to use the Carbon Copy Cloner.


Restart the Mac and hold down the option key until the Mac Startup Manager icons appear. Hold down the control key while selecting the Big Sur that is not shown as the current default.

An alternative would be to select the startup disk from macOS Recovery.


Remove the original. An example is given below. Assume the output from diskutil list is as follows.

The command to enter would be as follows.
diskutil apfs deletecontainer disk2


Reclaim the free space. An example is given below. Assume the output from diskutil list is as follows.

The command to enter would be as follows.
diskutil apfs resizecontainer disk1 0

The final result is given below.