does my macbook use smart charging even when charging over USBC?

The CHARGER is inside the MacBook Pro, and is completely under program control. Your Mac accepts some power when needs to run, and accepts more power when intends to charge its internal battery. A power source like the Power Adapter can not 'force itself' on your MacBook Pro. Only the algorithms inside your Mac can decide when is a good time to charge the battery.

Any external power supply that provides "USB Power Delivery" (including certain Docks and Displays) must negotiate over the USB cable using USB Power Delivery Protocol, and can not 'force itself' on your Mac. The Voltage and Current are delivered only after your Mac requests and the charger agrees to supply power under certain controlled conditions. The computer is in control of the entire process.

The classic way to brute-force improve battery longevity was to create a hysteresis -- to postpone staring a charge cycle until the charged state had declined to a lower level, such as 92 percent, and when topping off, stop before 99 percent. 

Catalina software 10.15.5 and later for MacBook Pro with T2 chip (2018 models and later) includes a feature called Battery Health Management. Now, based on your usage patterns, this widens the hysteresis to initiate a charge cycle at a lower level, and stop well before 99 percent. 

About battery health management in Mac notebooks - Apple Support

About battery health management in Mac laptops - Apple Support

Battery Health Management feature tracks your usage history and detects that you do not generally run your battery down to low levels. So when you see less than 100 percent as a full charge, it is charging less than the highest level for longest battery lifetime.

NOTE that charging to 80 percent is a side-effect, NOT the GOAL.

The goal is to have some hurly-burly around using your battery, either by your actually using it, or by leaving it less than 100 percent charged, so that the cells are not 100 percent charged 100 percent of the time. Battery Health Management will also from time-to-time let the battery decline slightly to 'get some exercise' and run lower (thought to be around twice monthly) if you do not do so by your ordinary use.

The feature tracks you actual usage. If you continue to not require 100 percent charge for a few weeks, it generally will revert to a lower "full" level. but it takes some experience, which takes some time

If your recent usage patterns demand top battery capacity, the battery will charge completely to be sure you are not stuck out in the wild with no power.

Nickholl wrote:

that for all the info! thats really interesting. Basically I used my laptop plugged in on my desktop for most of its life, vert occasionally unplugging it and using on battery. Does this mean I should occasionally unplug it to use on battery?

I liked the idea of using the USBC cable as power and data from my monitor as it free'd up the ports on my laptop. :-/

you do not need to purposely unplug from your monitor. Use it as you see fit, there is no harm. Let the software work for you no need to micro-manage the battery/charging.

The rule of thumb—if you are near the mains leave it plugged in, if you need the portability then run on the battery only—this is how you help optimize your battery charging and extend the working life of your battery by reducing the cycle count.