Depth of field is controlled by aperture, focal length, and distance to subject. The degree to which the subject is actually isolated from the background is also very important. This simulator manages to bury one aspect of this information and ignore the rest.
All else being equal, to increase subject isolation:
* Increase separation between subject and background
Each of these things is equally important. (But all else is not equal, so if you're determined to stand in one spot and not change your composition, just increasing aperture size is your only option.)
Yes, this is indeed the case! I shoot with a prime, so the ability to change the focal length doesn't really come to me. But I think it's very important that you brought up specifically the fact that the distance spatially between the camera, the subject, and the background are very important!
All else being equal, to increase subject isolation:
* Get closer to the subject
* Increase focal length ("zoom in")
* Increase aperture operning (smaller aperture number)
* Increase separation between subject and background
Each of these things is equally important. (But all else is not equal, so if you're determined to stand in one spot and not change your composition, just increasing aperture size is your only option.)