Explain how we can date rocks using radioactive isotopes. Where are these isotopes found?

Usually as secondary element substitution for a major element in a mineral. We don't usually have uranium or thorium minerals in rock, say. But potassium (K) does occur as a major element constituent in the minerals we date, so it cuts both ways. Strontium can be either. Carbon that we use for dating is not typically in minerals, though, but is in organic matter instead (by the time it mineralizes, the C14 is gone).

The radioactive isotopes are found with the non-radioactive isotopes of the same element. Asking me where elements are found is kind of an odd question. Look around, everything you see is elements. Individual elements have their own specific behavior and tend to be found in settings that are specific to the element. cannot generalize.

You are going to have to explain how we measure concentrations of parents and daughters on your own. The basic idea of dating is that if we can know or figure out what concentrations of parent and daughter were present at the start (when the thing was formed), and we know the rate of decay, we can calculate how long the decaying had to have been occurring to get the concentrations we can now see.