Nowadays you have to install the b43-firmware pkg from the AUR to get those old broadcoms working. To install without net search the forum for ‘pamac installing local package without internet’ or similar.
After that’s done run:
sudo modprobe -r wl
sudo modprobe b43 allhwsupport=1
and you should have wifi.
However, this may not survive a reboot. Have a look at the solution of this post Macbook Wifi not working - #7 by Lalo to make it permanent.