Just to clear things up a bit.....
There is GPS and A-GPS (assisted). Most smartphones have A-GPS which you can think of as 'enhanced' GPS. An A-GPS receiver has an extra component that pulls ephemeris data off the mobile network. This allows for a quick location fix, and allows for rough positioning inside buildings, tunnels, etc. Most A-GPS receivers (in modern day smartphones, anyway) are 'true' GPS receivers in that you can lose the cell signal and still pull a location fix from the sat signals under open sky, or near windows indoors (just like your Garmin or TomTom). The TF has a 'traditional' GPS receiver (not assisted) as there is no cellular modem in the TF (although future TF's will likely have a 3G option, and thus may have A-GPS). Thus the TF requires you to be near windows when indoors, or ideally under open sky for an accurate position.
Yes, the TF will get a sat lock inside a car (why wouldn't it?) in the same way your Garmin will. Multiple Youtube videos on this.
No, most (commercial grade) Nav apps don't require you to be online to navigate, as they have map data that you download and install to your SD card. I use CoPilot ($5) for offline use, as a backup to GMN.