No command works in Terminal

You will need to create a .zprofile and .zshrc file, the first for Terminal login, and the second for interactive usage. Until you get your PATH set in zsh, you won't be able to do much, or find commands in any path.

I have the same content in both of these dot files:

export PATH=".:$HOME/bin:/usr/local/bin:$PATH"
autoload -U colors && colors
MyHost="$(/usr/sbin/networksetup -getcomputername)"
PS1="%{$fg[blue]%}${MyHost}: %~ %{$reset_color%} %% %{$fg[white]%}"
export EDITOR="/usr/bin/vim"

You can use TextEdit in plain text mode to save these dot files into your home directory, and dealing with command-line Terminal editors and wonky paths will not be fun. Then quit Terminal and relaunch it and zsh should read the .zprofile and set your PATH so you can actually reference commands without the explicit path to them.

That looks like zsh prompt, however there is no default "help" command in zsh

% help

zsh: command not found: help

you can verify your shell, copy and paste:

echo $SHELL

You can always see more over the man page, copy and paste:

man zsh | more

or return to something more familiar, like the previous

cat /etc/shells

/bin/bash

/bin/csh

/bin/dash

/bin/ksh

/bin/sh

/bin/tcsh

/bin/zsh

to change back to /bin/bash for example:

chsh -s /bin/bash

Here’s an introduction to zsh, for starters.

zsh has replaced bash as the default command shell, starting in macOS Catalina.

The old bash from earlier macOS releases remains available.

You can change the shell, if you prefer a different default shell.

There’s some documentation on Terminal.app available, too.

Expand the Table of Contents on that latter web page for yet more.