This will define an alias called find-and-activate that will search for directories within the current directory (.), and then check if each directory contains a file called activate.fish. If the file is found, the . command will be used to execute the file in the current shell. The return command will exit the alias once the first activate.fish…

config.fish

# Define the function
function find-and-activate
  for dir in (find . -type d)
    if test -f $dir/activate.fish
      . $dir/activate.fish
      return
    end
  end
end

# Define the alias
alias find-and-activate='for dir in (find . -type d); and test -f $dir/activate.fish; and . $dir/activate.fish; and return; end'