There are two questions here:
Will using a non Apple USB-C charger harm the computer/battery?
No, as long as the charger is from a reputable supplier there is no reason to expect any physical harm from using other brands of charger, power is power, and the MacBook will only accept the amount of power it needs to fulfil it's need.
Will keeping my battery topped up at all times, no charing/discharging harm the computer/battery?
Yes and no, this is a particularly complex subject, but sufficed to say your battery like a person needs to remain active to be healthy, but equally overdoing it is going to wear it out prematurely.
So your device has a rated number of battery cycles, typically about 1,000 on modern devices. A cycle is one full discharge and recharge in however many little bits that takes, so limiting how much you discharge and recharge the device can prolong the life.
However the battery needs to remain active, and keeping it fully charged all the time can do harm as it's constantly keeping the battery topped up...
There is a really good article from Apple which I don't want to reproduce here that you should read: