Hi - There are a couple of reasons I can think of why you might not have all the sounds in GB that you may have seen elsewhere.. - That video is for GB iOS, which has some features [and possibly some sounds] that GB for macOS doesn't have [that whole string fretboard is a GB iOS feature specifically, AFAIK] - People who have GB on Mac and also own Logic will be able to access [some] Logic instruments within GB that come with Logic but don't come with GB by itself [including Cello] - GB can access third-party instrument plug-ins, so sometimes you may see an instrument in a video that uses a third-party plug-in that doesn't come with GB and that you don't own Btw, Smart Strings & String Ensemble are the only string sounds I see under Orchestral > Strings in GB as well, but since I also have Logic installed I could load some of Logic's strings in GB [if I wanted to] -- those sounds show up under Legacy > Logic > Orchestral > Strings. However if you had Logic installed why would you be using GB..

Yes, he's suggesting that if you pay $30 for MainStage [rather than $200 for Logic] that MS will also come with the same sounds/instruments that come with Logic, and once they're installed you'll be able to access them in GB. I CAN'T confirm that, because on my system those sounds were installed as part of Logic, but I suppose if you don't mind springing for the $30 for MS you could try it, to see if it makes the sounds you're looking for available in GB..

The MainStage manual mentions the Studio Strings instrument, which [according to the manual] 'provides a collection of highly expressive single and section stringed instruments'. So I guess you'd get that with MainStage. HOWEVER, I DO have those instruments installed and they DON'T show up in GarageBand on my system, so I don't know if that would work for you -- it seems that [despite what that guy in the video said] GB might not actually support all the instruments that come with Logic or MainStage.. Btw - AFAICT that Smart Strings feature you saw in the video is a GarageBand iOS [iPhone & iPad] feature -- I can't find any indication anywhere that it was ever added to GB macOS. You might want to look around for third-party string instruments that you can use -- as long as they include an AU [AudioUnit] version they should open and play in GarageBand macOS. If you search 'virtual strings' and virtual strings free' you should find a whole lot of virtual string instruments that start out at around $30-$50 [and up], including a bunch of free string instruments from companies like Spitfire and others..

Spitfire usually makes good instruments. Download their free instruments (and/or trials?) and check them out..