How to reduce clock/voltage of macbook pro 2018

The USB-C Apple laptops do get hot and will automatically throttle the CPU if the cooling system is unable to keep the temperatures within normal operating range. CPU temps normally run between 65C to 90C for most work loads and is perfectly normal. If the laptop is hot even when you are not doing much on the laptop, then you most likely have some third party software installed that is causing your problem. To look for a software issue run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper.

You can also run the Apple Diagnostics to see if any hardware issues are detected.

You can also try an SMC Reset, but I doubt this will help.

That's the bandaid approach and will likely not fix your problem. You need to determine why it's overheating: clogged air vents, tight fitting cover, using it on a soft surface such as a bed or pillow and etc.