Why can some commands be executed when the /usr directory is not visible?

Cause

Shells such as bash and zsh have built-in commands.It was also found in Bourne Shell, the base of bash.The built-in commands do not refer to /usr/bin and are available even if /usr is no longer available.(If the shell is moving)

embedded commands

A list of built-in commands can be found using the help command.
You can also use the type command to determine whether it is built-in or external.

$type cd
cd is a shell built-in function.

$ type man
man is /usr/bin/man

$ help
GNU bash, version 5.0.17(1)-release(x86_64-pc-linux-gnu)
These shell commands are internally defined.Type `help' to see the list.
US>Enter `help name' to provide a more detailed description of the function called `name'.

~ (abbreviated below)

Reason for Existence

There are several reasons why it is a built-in command.Accelerate, or even if you can't execute it without a built-in command in the first place
(For example, the cd command... because external commands will revert after execution.)

There are other examples besides shell built-in commands.

When /usr is full, I would like to put it in single user mode and replace the /usr mounting destination. Of course, /usr does not exist in the middle of replacing the /usr mounting destination.You can still use the commands in /bin or /sbin under the root partition.

I mean, the various commands stored in /bin and /sbin are made with static links to work without /usr (at least in commercial UNIX)

Most field-built binaries are ./configure to use the following shared libraries, so you need to have /usr to install them, so you should not use /usr/local or /opt to overwrite /bin.