switch_brew.fish

# Usage:
#   $ switch_brew (uname -m)
#   $ switch_brew x86_64
#   $ switch_brew arm64
function switch_brew
  set -l arm_brew_path "/opt/homebrew/bin"
  set -l switch_to $argv[1]

  if test "$switch_to" = "x86_64"
    if set -l index (contains -i $arm_brew_path $PATH)
      set -e PATH[$index]
    end
  else if test "$switch_to" = "arm64"
    set PATH $arm_brew_path $PATH
  end
end

This file should be added to ~/.config/fish/functions/switch_brew.fish. Then add this line to your ~/.config/fish/config.fish file to automatically set the correct brew on startup:

switch_brew (uname -m)

See homebrew's installation instructions for more context, or this explanation.

Additionally, this blog post goes into more explanation on the multi-brew setup.