What you refer to as "high voltage" would be properly called line voltage. The vast majority of light fixtures are line voltage, which means they use the standard 120 volt power as it comes to your house. Any fixture that takes a bulb with the familiar screw-in type base is going to be line voltage.

If you had undercabinet lights with xenon bulbs, those would probably be low-voltage, meaning they use a transformer to cut the standard 120 volt current down to, perhaps, 12 volts. Also, if you have very small accent spotlights that use mr16 reflector bulbs, or track lights with modern, compact little fixtures - those could easily be low-voltage. Mini-pendants with tiny bulbs are also likely to be low-voltage.

As to your #3, I don't believe it's legal to wire a dimmer to an outlet.

Lutron has the only approved way to install a wall mounted dimmer for a receptacle and you must use all of the UL listed parts including dimmer, receptacle and plug. The link shows what's needed.

Here is a link that might be useful: How to dim a receptacle

Thank you so much for the information. I did not know about #3. Now I can get the appropriate dimmer switches.

After getting the responses, I thought all I had to do is get line voltage dimmer switches, but I got a message from my contractor that I should get all low voltage dimmer switches. I live in an apartment in NYC, would that make a difference to the standard 120 volt power mentioned by jon1270?

Before going dimmer shopping you must know what you need. Here is a link to help you figure that out.

A "contractor" should not be sending you out to purchase dimmers without a list of what to get. You get to choose between styles and colors but not type. The fixtures choices have already chosen the type. Magnetic low voltage dimmers can dim normal incandescent but they are much more expensive. The list of fixtures you provided could be but are not typically low voltage.

Here is a link that might be useful: Dimming basics

dim4fun - Thanks for the link. It was helpful

It turned out I misunderstood my contractor. He meant that all the fixtures in my kitchen which he is installing are low voltage. He will let me know about the rest of my apartment.