The transit app does utilize motion sensor data to try to guesstimate your position while you are in subways or other areas with low GPS reception.
Here's the privacy policy: https://transitapp.com/privacy-policy (Aug 14 2025 archive version). They have the who, what, where, when and why and how your data is used all in the policy. It's quite readable, and on 3rd party data sharing, they list out what they collect and which third parties they give it to.
The app is made by a company based in Montréal, Québec, Canada. Personal information data is stored in Canada and the EU, location data not tied to your personal info is collected and stored in the US. The company has at present committed not to sell collected info to databrokers and advertisers, but they do provide it to researchers and transit agencies.
My privacy model is that if I am to consent to my data being collected, I need to receive a tangible benefit that is directly related to the data given. So location advertising to help myself and other people around me know where the bus is is an amazing benefit all around. So I use that feature.