I have a Nexus 5 phone (arm) with Android 9. I have no camera app in Lightroom application. In the lower right corner, I see blue add photos button but it is now accompanied by a camera icon... The camera works in this device - I have a google camera app, which works without problems.
We are sorry about the experience with Lightroom mobile.
Alot of the features of the in app camera in the Lightroom mobile app depend on the specifications of the device you are using. If the device does not meet the requirements the camera feature may not be available.
Please go to the sidebar menu of your mobile device, then go to Device info and storage and select Device info. Please share the information mentioned with us here or a screenshot of the information displayed.