From app blogs [Methodology](https://www.google.co.in/search?q=open+signal+app+gives+wrong+information&oq=open+signal+app+gives+wrong+information&aqs=chrome..69i57.33827j0j4&client=ms-android-motorola&sourceid=chrome-mobile&ie=UTF-8#xxri=3) explains how they collect data


* Crowd Sourcing (already pointed in comments)


* Monitoring users who installed app continuously  

So, the accuracy pretty much depends on how many people use the app with GPS *on*


* Data Analytics

Also see [Cell towers/wifi routers not located correctly](https://opensignal.com/blog/cell-towerswifi-routers-not-located-correctly/) which is dated but relevant

This app was recently launched in India and I noticed this

1. Cell towers are very well mapped of a  provider, though, in my area tower density is low ( likely because they made it open source as a part of publicity )

2. Cell towers are not well mapped, for another provider,  though the tower density is much higher

Not a very dependable app , IMO