I was under the impression that Doodle3D just allowed you to draw in 2D and then turned into 3D by basically extrapolating the 2D planes you drew in the program. The whole point is that you are not modifying a 3D object and do not need to know how to 3D model. It doesn’t load STL files, it just exports them from whatever you doodled.
It’s not that it isn’t an active function yet - it’s not the intended function at all.
As for a different program.
Since you’ve given no details about what or how you are planning to modify - I guess start with TinkerCad and see if it can make the modifications you are looking for (for most quick mods it’s great and the learning curve is very low).
If it doesn’t you are probably going to have to invest a bit more into learning how to model 3D and experimenting to see which program works best for your goals and thought process using meshmixer, one of the CAD variants (openscad, freecad etc), blender etc.
SketchUp can technically do STL with an extension - which has its pros and cons.
Note: While I have used the above programs, I don’t generally speaking - so I’m not a good person to ask specific “how to questions” for them, so assume my opinion comes with a GIANT grain of salt. “Simple” tends to translate to “limited” in my experience and then I get annoyed - I’ve used Lightwave 3D and Maya for over a decade (not just for printing obviously and not just for modelling) and that’s what I’m comfortable in. Neither are simple!