KoboToolbox provides a GUI frontend form builder (technically I think it is/was referred to as ‘KPI’), which produces XLSForms, which in turn are converted to actual XForm form definitions using another tool called pyxform. Alternative XForm builders are ODK Build, or using Excel/Google Sheets to write your XLSForm spreadsheet, and then converting it into an XForm running pyxform on command line or using a GUI tool like XLSForm Online, installing an app like XLSForm Offline to do it (both of which are just pyxform wrappers), or upload it into KoboToolbox as an XLSForm and export it as an XForm.
Or there is nothing stopping you from writing raw XForm HTML using your favorite text editor 
Once into an XForm format, you can use a web-based XForm renderer like Enketo to view it or fill in your form (BTW Enketo is what Kobo uses for preview and web form filling…), or use an Android mobile app XForm renderer like ODK Collect, or its ostensibly rebranded KoboCollect app. [there are other ODK XForm compatible apps out there, like my iXForms for iOS, but they’re not ready for public consumption yet…]
There are a few other commercial solutions that use XForms under the covers, eg Survey123 (Esri), SurveyCTO, Orbeon, … which often also use pyxform/XLSForm to generate XForm form definitions, but oftentimes these introduce custom features so aren’t usually 100% compatible with the ODK toolchain.
The main different between Kobo and ODK is that Kobo provides a complete end-to-end hosted solution, and has its own custom backend services for receiving and processing form submissions, whereas ODK just provides the various components with which you can build your own custom solution, using either ODK Aggregate or ODK Central for a backend server.
Hope that helps! (and that I’m not being overly biased… Disclaimer: I’m on ODK TSC 
)
