There is no one specific answer.
At the school district I work for, most of the teachers use their school issued laptop alongside a second monitor. They use one display to present whatever they want and the other one to see their students.
I would look for:
At least an 8th generation Intel Core i5 CPU (or AMD Ryzen CPU)
At least 8 GB of RAM
At least 500 GB of SSD storage
Consider buying a second display, especially if you’re going to be doing live teaching sessions.
A web cam if you’re using a desktop.Though, they may be hard to come by currently since there is very high demand for them.
Ethernet connection over Wi-Fi.
For the most part, a laptop or desktop will work fine for teaching online. It mainly comes down to what works best for you. Desktops are great if you don’t plan on moving your setup at all, or rarely. Laptops work well if you want to be mobile.