you can't - it's just a shrine to honor the protectors!
You can get them in the underground. Go to the old man in the house next to Eterna PokeCenter, and he gives you the explorer kit, which can be used to go undeground.
there aren't any Pokemon inside the house
You can only catch one on route 216 with the PokeRadar in a swarm. You can see what Pokemon are swarming on a particular day by talking to Dawn/Lucas' younger sister at their house in Sandgem.
That is for decoration.
You don't actually get the magma stone but once you go to that guys grandpas house, he'll tell you about HEATRAN, then you go back to stark mountain in the room Cyrus got arrested and heatran should be there
the house to the top left corner of the Pokemon center in nimbasa (middle city)
The house in the middle of celectic town is a home of is a home a lengendary Pokemon named latias and latios. by Daniel carreras
You have to have connected to Wi-Fi a long time ago. If you did, go to the city with the boats and go up to the house that was locked.
You can't
go to cianwood city and go to the house in the bottom middle.
I think it's just a shrine to the legendary Pokemon and that you cannot get into it in any way.
That little box thing is a shrine to whom I may think celibi