Where grease is needed can be a surprisingly controversial topic. You develop a personal approach over time, especially if you work on bikes that were badly maintained. Then you see what can happen in specific places without grease and correct it.

If i had to propose some rules of thumb with exceptions, it would be:

To Grease:
Threaded parts, like or unlike metals. This is to prevent seizing due to corrosion but also because torque specs assume slightly lubed threads. Otherwise the friction coefficient is too high.

Unthreaded, like or unlike metal interfaces where removal is important and/or leverage is bad. Aluminum seat posts and steel frames are ripe for galvanic corrosion and get stuck. Also stem quills.

Not to grease:
Like or Unlike metal surfaces that shouldn't move, or where leverage is good if you have to remove the part. Handlebars to stem, ahead stem to steerer. Saddle rails to seat post clamp.

Check the user manual carefully and proceed at your own risk:
Metal, carbon, composite, or plastic interfaces.

Surprisingly contentious
Whether to grease press fit metal interfaces such as headsets or bb spindle tapers. I always do. Others don't, and that's fine with me.

Italian bottom bracket fixed cups, but this is a really specific and uncommon issue. Some people grease and really torque it down, some people use thread locker. i use grease.