buying tickets with a family

The pass is your ticket. You can have multiple passes on one phone.

Pass and reservations are separate. Reservations can't be booked in the Rail Planner app. It can only forward to some of the website where you can book, but it doesn't always give the best one, and it has a preference for interrail.eu, which has a number of disadvantages, such as added booking fees.

Therefore just ignore the Interrail website for reservations and use this page as a guide:

https://www.seat61.com/interrail-and-eurail-reservations.htm

I thought that I had to book tickets before boarding trains? Is that not right? Can we just get on a train and show our pass?

The pass is your ticket. You don't need to book any tickets for trains included in the pass.

You do need to add each train you use to your pass. After you've done that, you can show the QR code for the day, which is the ticket. Also see:

https://www.interrail.eu/en/interrail-passes/interrail-mobile-pass/getting-started

For some trains reservations are mandatory. For some they're optional. For most trains reservations aren't available and you travel on your pass only.

This general introduction may also be useful:

https://www.seat61.com/how-to-use-an-interrail-pass.htm

Thanks for this response. In step 7 of that guide it says:

Step 7, when you board a train, toggle that train onto your pass

Then it goes on to say:

Incidentally, toggling a train onto your pass does not in any way 'reserve' or 'book' that train.  Reservations are an entirely separate thing.  All this does is (a) make the pass valid for that train, it's not a valid ticket until you have done this

This is the part I am referring to. Not the reservations.

That is indeed what you have to do. But that's not "booking a ticket". It doesn't book anything. It just registers the train you're going to use. It's the digital equivalent of writing the train in the trip diary of a paper pass.

Ok, wrong language. 
so how do I do that for al the different passes to generate qr codes?

Yes, you need to do that for all passes. Each pass needs its own trip.

If all passes are on 1 phone, you can add a journey (containing 1 or more trains) to multiple trips in one go. You'll then need to toggle the journey for each pass individually.

Do keep in kind that, if you have all passes on 1 phone, you can’t travel separately.

Before you board a train you must have the train added as a Journey and connected to a Trip in the Railplanner app and the train must be activated by turning the switch on. You can add the train well in advance but wait with the activation until just before boarding. 

So is it best to make a copy of my trip for all 6 passes?