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

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.

The largest problem with tonearm bearing is too tight. Even a tiny tiny bit too tight is huge pressure in very small areas. Just on the verge of loose is way better than failing to understand even a little tight is way way too tight.As for damaging them mounting a cartridge? (if you are using an automotive air gun?? probably.) Otherwise not likely unless you are ham fisted Mr Bungle himself.

VPI arms can be unplugged and lofted off......easier than easy...no damage.

That's why I like my Dynavector 501 arm, with the removeable headshell. Myself, I'm always worried about damaging the stylus/cantilever when mounting a cartridge. Adds to the challenge, I'm glad I don't have to worry about mounting a $15,000 one!

Regards,
Dan