If you do nothing the save is performed to the Lightroom database, and from that point forward (unless you undo it) the image shown in Lightroom has your edits.
To make the change permanently, by destructive save, would cause problems in Lightroom. You could overwrite the existing JPG with the new JPG, but then Lightroom will apply the edits to the edited/saved JPG. For example, if you increased the exposure on the original JPG by +1.0, and then did a destructive save, then the file on disk now has exposure increased by +1.0, and Lightroom then applies the editing information from its database and increases the exposure by +1.0, giving a +2.0. Similar issues would happen if you straightened a horizon and then destructively saved, the image in Lightroom would appear to have the angular change applied twice.
So while this can be done (although it involves an export step), the negative consequences are quite severe, and I would urge you to rethink your need to do this. As long as I have been involved in Lightroom, I have not heard a good reason to do this, nor am I aware of anyone doing destructive saves. You would be much better off understanding how to use Lightroom as intended.
If the issue is actually what export folder to use ... this is simply handled by most people by making an export to a temporary folder (I always use the same folder named Temp). All my exports go there, so I always know where the exports are. Once I am done using the exported photo (for example, I have e-mailed it or uploaded it), I delete the exported photo, knowing that I can always re-create this export exactly if necessary.
Lightroom is a non-destructive editor that saves all of your edits in the LR catalog, along with keywords and other metadata you can use for "organizing" you images. You can create multiple virtual copies of any image and apply different settings to them without exporting to separate files. Collections are also virtual copies that can be used for further organization. Some good tutorials here:
Peter Krogh has an excellent eBook on organizing your photographs with LR5:
One of the main tenets of Lightroom workflow is that you simply do not touch your originals ever. Do not look at them as the final product but simply as the raw "negative" that you keep safely tucked away. There never should be a reason to find your originals except perhaps to move them off to an external hard disk if your main disk is getting full.