Time to visit the manual again for you

1) So the instances in these 2 nodes belong to the same SID ?
No, each has its own sid but the same database name

2) The database in the shared disks are actually the oracle binaries and datafiles and other physical files?

Typically the oracle binaries are not shared, they are local to each server

3) At any point of time, there is only one active instance among these 2 nodes?

no, both are active - they share the load an dprocessing

4) Finally, if one node has some problems, the other instance in the other node will take over ?

if you have it set up correctly, your sessions can failover the other node. So yes it will take over