Hi SamJubilee , Cartesian coordinates can be done in both 2D sketching and in 3D mode. There are some methods unique to each, and some methods that work in both.
Subashni 's example it is in 2D. After turning on cartesian dimensions, it asks for the basepoint as mentioned. You can also hold SHFT and click a point to move the dimension basepoint to another location, or there is a toolguide for that too.
Another method is to add the point in 2D or 3D just by turning on the point tool and click somewhere (for 3D, after clicking the point tool, click the 3D mode button). then turn on Move, select the point and use move to specify the X, Y, Z location. Works in 2D or 3D