How do laser rangefinders work when the object surface is not perpendicular to the laser beam?

Some laser rangefinding uses a retroreflector, which will bounce the laser light back in the direction it came regardless of orientation.

Otherwise, lasers operate at a very specific frequency, so the signal/noise ratio only needs to be strong enough to be detectable at that frequency.

If you shine a normal laser pointer on a wall, even if the wall is pretty far away, you can see the spot it makes. That means your eye can pick out the reflected laser light. The electronics can be made better than your eye, so it's not too hard to see reflected laser light..

so only a tiny fraction of beam energy is reflected back to the device.

This tiny fraction is enough. With respect to ambient light: One can modulate the laser beam, and filter the the voltage of the receiving photodiode for this modulation frequency and phase. Another precaution is to have a light filter in front of the receiving photodiode which only lets the wavelength of the laser pass. I think both precautions are used. And of course the receiving photodiode is focussed to a spot of some centimeters diameter around the laser spot. Try to point the range finder to a mirror, in that case the range finder should fail, exept the mirror happens to reflect precisely back to the range finder (which is rather unlikely). Reason is that from a (clean) mirror You don't get a stray reflection.

The amount of laser energy reflected back will be the limiting factor of its effective range. However, since the laser's radiation is of a specific wavelength, it won't be confused by extraneous radiation from ambient sources.