All the standard Detachments have the restriction that the units within must be from the same Faction, yes. This means for example that an Imperial Guard commander can't be the HQ choice for a Patrol Detachment otherwise made up of Eldar troops (for obvious reasons. Why would they listen to the Mon'Keigh anyway?) or vice versa.
You can build your Battle-forged army out of any number of Detachments, and the restriction is purely on the Detachment itself. So you could have, for example, a Battle-forged Army made up of the following:
1. A Tech-Priest Dominus with some Rangers that form a Patrol Detachment
2. A Chaos Sorceror with 2 squads of CSM in another Patrol Detachment
Due to the Faction restrictions on the Detachments, what you can NOT do is put all the above together in a Batallion Detachment. So in exchange for increased freedom and more unit choices you might miss out on some Command Points (the above example is specifically picked to illustrate that).
There is no longer an Allies Table, so you can mix any Factions you want together into your army - even the Tyranids are now free to ally with whoever.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.