That depends on the sensor of course. If it is a sensor that sends digital data, you would use the UART (or SPI) .
If it is analog, the ESP has 1 analog input (ADC) which has a 10-bit resolution for an input between 0 v- 1v.
This means you should prevent applying more than 1v to it.
Only 1 ? Yes only 1, but you can use a few digital pins to power a few sensors, powering 1 at a time and reading the result.
I would host a webserver on 1 ESP which will serve as an AP.
All other ESP's can connect to it in STA mode. They can 'POST' to a page (or a GET request to the ESP it doesn't matter much unless you specifically tell it to distinguish, GET is easier to debug), and this data can be extracted by the server, updating variables that can be accessed by the App when it visits that same or some other page.
Of course you can do packets of some other kind, which will probably be more efficient, but this is what i would do. Keep in mind that the maximum of connected devices is 8.
Yes you can of course, it would need to send it anywhere and can just incorporate the data in the page it is hosting for the App.
Normally one would start with that. Interfacing the sensors with the ESP is the next step, and to just include the results in a page that you already can connect to is the way to do this.