How to add a file to /usr/bin so I can access this from any folder via Terminal?

The /usr/bin location on Catalina and later is now on a read-only System volume that disabling SIP will not gain you access too. It is off limits.

Why not create a bin directory in your home directory (~/bin) and put the assembler in there. Then modify your PATH statement in your respective shell dot file:

export PATH=".:${HOME}/bin:${PATH}"

and then make it current for your relevant shell dot file where this PATH resides:

source ~/.zshrc
source ~/.bashrc
source ~/.bash_profile

and now, any time you need that assembler, regardless of the current folder, it will be found by the shell whether Zsh or Bash. I use the leading dot so I don't have to do "./something with scripts or files.

PATH=~/Users/colinjoss/bin:PATH

is wrong. The $ is missing in front of $PATH

export PATH=".:${HOME}/bin:${PATH}" 

would be better, assuming you also want to be able to execute programs and scripts in the current working directory, which is what the .: is specifying.

If you do not want that, then

export PATH="${HOME}/bin:${PATH}" 

will do the trick.

As an alternative to PATH (which will work), you could add the assembler as an alias:

https://scriptingosx.com/2019/07/moving-to-zsh-part-4-aliases-and-functions/

/Users/username/.zshrc

Thanks Bob.