The building of fillets and angle mates in a flat or a 3D object in AutoCAD is done with a team MATE (_fillet). You can run the command by typing at the command prompt or by clicking on the toolbar.
After running the command, options for further actions appear:
Entering a letter:
T — cancel the command,
And it will be possible to select a polyline where you will build a fillet all corners of a given radius,
D — set radius value,
B — set the mode with or without cropping the lines that create the angle to be rounded,
H — you can round multiple corners without interrupting the command.
If the choice of the lines forming the angle, but does not have a common point of intersection to hold down the Shift key, the command will extend these lines to their intersection point.