A Foci point of an Ellipse is easily constructed from a circle of the radius of the major axis, with its center relocated to the end of the minor axis.
(or intersection of minor axis tangent.)

It may be helpful to do a cplane on the center of the ellipse, aligned along an axis of the ellipse.
Helpful points are the center point, the major axis end, and the minor axis end.

Two similar references:

https://en.wikipedia.org/wiki/Ellipse
(see section: "Construction of a directrix")

https://math.stackexchange.com/questions/1187375/given-a-drawing-of-an-ellipse-is-there-any-geometric-construction-we-can-do-to-f#:~:text=Pick%20the%20endpoints%20of%20the,circle%20with%20the%20major%20axis).

- Brian