i never get the popup about usb connection when i connect my moto g 3rd generation android 6.0 to pc. due to it, I am not able to connect my phone to any pc or laptop. it is in the default charging mode.
The issue is not your phone or pc. You need to change your usb cable and install a usb driver from the respective brand.
Marshmallow, on the Moto G 2015 at least, does not give a pop-up like Lollipop did, you must manually grab the pull-down menu and select USB Charge Only and switch it to MPT each connection.
There is an option in Developer Options to set the default USB operation, but the setting seems to have no effect on operation. As of the current MM release it isn't known if this a bug or as designed because Moto hasn't made an official comment on it.
It's a known issue. Google wants you to use their cloud and not access Android like its a USB drive. It's stupid and nothing more than vendor lockin, because if you're in the situation I'm in, I can't get my files off my phone except for slow methods.