The battery cannot be replaced. However, the built-in battery should last around 10 years. After 10 years, the service life of the sensor has also expired (contamination from dust) and should be replaced. Therefore, the entire device should be replaced when the battery is empty (assuming the built-in battery actually lasts as long as discussed).
The sensor can be connected to the WLAN and also sends a message to the smartphone in the event of an alarm. However, the smoke detectors cannot be networked so that if smoke is detected in one detector, the alarm is also triggered in the other detectors.

A 10-year battery is installed, which officially cannot be changed. I assume you can change it anyway (the device seems to be easy to open). The connection is made via WLAN. The detectors are not permanently connected to the WLAN, but only establish a connection when they detect smoke.