Where is the data( temperature and pressure ) being stored in microcontroller?
Short answer: Where your program code provide storage for those.
Code you write in your (main) program, or a function you write,
need to define storage for temperature and pressure like this:
those lines need to be before the place where those variables are used.
Also, before that again, your code will need
if those lines are not in place already.
The ampersand '&' before a variable names in function calls,
is indicating to the compiler that location (address, pointer) of the variable shall be used when function is called,
such that the function can fill in the (new) value in the storage your code have defined.
For information about how the sensor work, you will need to read the Datasheet for the sensor:https://www.bosch-sensortec.com/products/environmental-sensors/pressure-sensors/pressure-sensors-bmp280-1.html#documents
open the 'Documents' tab in the webpage, and download or open the Datasheet document.