What are the similarities between a unitary and federal government system?

There are not necessarily any similarities at all between these two systems other than the fact that both have governments.  These two systems can be quite different.

In a federal system, power is shared (by law) between a national government and local governments (often called state or provincial governments).  Neither level of government can legally take power away from the other level.  In a unitary system, things are different.  The national government is the only government that has the right to exist.  The national government can create local governments and give them powers, but it can always take those powers away.  The national government, in this system, is all-powerful and the local governments (if they exist) only have whatever powers the national government decides that they should have.

From this, we can see that these two have little in common.  Both systems can have national and local governments, but they do not have to.  Both systems can be democratic, but they do not have to be.  The only thing that they have to have in common is that they both have to have a national government of some sort.