123 was built on the Qt Framework in the past but it is currently being rebuilt in .Net as Qt is end of life.
If you are going to build an app you would probably want to use the SDKs from Esri. See here https://www.esri.com/content/dam/esrisites/sitecore-archive/Files/Pdfs/library/fliers/pdfs/arcgis-ru... This can get you going much faster and can use AGOL as the backend so you can cloud sync data.
It will be a ton of work though. 123 has a 9 year or so head start on you and a team of developers. The last two companies I have been at we developed our own apps using the Esri stack specifically to have better offline support (this was starting back in 2013). Both of those apps have now been sunsetted and we moved to Field Maps. Partly because now the offline support is better but mostly that it was just so hard to keep up with advancements at the pace that Esri can do them. Again its one dev vs many. Also it was really hard to keep devs around before they got a new job. Not trying to discourage you, just be realistic. I would find something 123 or others do not do well and focus on that. Data entry speed, esp in repeats is one idea. Or industry specific like MobileMap by MBG.
I would also research what is out there now as there are a lot. I did my masters thesis on all the mobile data collection options out there but that was back in 2011 so its way out of date now.
Good luck
