[fngjdflmdflg]

You also need to check "Fixed aspect ratio" in the Ellipse select options, otherwise you will get an ellipse instead of a circle.

[eitland]

Or you can use the right modifier key.

Haven't used Gimp in a long time, but try holding down ctrl, shift or alt while doing the circle (or rectangular for that matter) select.

One is for aspect ratio, one is for center from starting point and the last I cannot remember.

IIRC the same goes for combining selections: modifier keys can be used to add, subtract or make intersections.

[geenat]

I just use shift, but that's photoshop muscle memory for ya.

[fngjdflmdflg]

I tired that originally and it did not work. The issue was that I pressed shift before clicking. After testing, MS paint and Photopea both support clicking shift before starting the selection to create a circle. Krita does nothing if you hold shift before selecting. I don't have PS right now so I don't know what that does. Seems unintuitive to me.