Typically, the clutch as used for manual transmission cars is activated by cable or hydraulics. On the 1987 BMW 325 and probably many other E30 models, the clutch uses hydraulics.
It uses the same fluid (and fluid reservoir) that the brake system does.
The slave cylinder for the clutch is on the driver’s side, inside the bottom of the transmission tunnel. To detach or attach to the metal hose that goes to the rest of the car, undo the 11 mm nut and the 14 mm nut. Whenever you remove or replace the manual transmission, that’s part of the job.
Breaking that connection lets air into the hose, and before the clutch can work again, it needs to be bled. To do this, remove the dust cover from the bleed nipple. It faces diagonally downward and is loosened with a 7 mm wrench. I recommend using a commercially available “bleeding bottle and pipe” set. It’s tricky work and is best done by two people: one below the car, tightening and loosening the nipple and the other moving the clutch pedal up and down on command.