tonearm bearings. How do we attach cartridge without ruining bearings?

I’ve never given it much thought. I’d think that you would have to be pretty ham fisted to do any kind of damage. 

If the bearings were that fragile, tonearms would wear out after a year or two of regular use.  In most cases, they are not.  I am more worried about the snowstorm we are now having.  Obviously, this assumes you take normal precautions not to apply too much rotational torque to the bearings whilst tightening the headshell screws.  Since the screws that hold the cartridge to the headshell rotate in the horizontal plane, if you just immobilize the arm wand while tightening them, I cannot imagine doing any harm.

Many times arm bearing problems are due to the relaxation of the metals of the given pillars, over time.
The metal relaxing will either tighten, or loosen the bearings.

This is so pervasive, that with Dual turntables, the bearing tightening or adjusting procedure is plainly spelled out in the service manuals.  ALL dual turntables need this. When done, and it is fairly simple to do..when done, they go back to being superb budget turntables.

Not all arms suffer from it, but some have a little bit.

Other than that, they can be fairly robust, but one can flat spot the bearings, if not careful.

But, yes, indeed, in most cases they are fairly tough.