Found this online from an England supporters website:
All of the tickets in your order will be delivered to the lead booker via a mobile phone app. The lead booker must download the app and then register to use it. The personal details submitted when he registers will include his email address and this is the flag or identifier which tells the UEFA computer which tickets to download to him.
The lead booker will need access to a compatible smart phone : an iPhone or Android phone, but it doesn't necessarily have to be his phone. It could be the phone of another member of his party.
The lead booker then transfers the other people's tickets to their smart phones using a 'transfer' option in the app. This step is optional. It is perfectly OK to leave their tickets on his phone if they do not have a smart phone. It just means that, at the turnstile, the phone will have to be passed between members of the group.
Note that the UEFA computer keeps an audit trail of transfers. So, potentially, UEFA will know if anyone tries to transfer a ticket to someone who was not a guest named on the original application.
As far as I know, you cannot have the same ticket resident on more than one phone to cover the risk that your battery might go flat.
A screenshot of the ticket will not work at the turnstile. The tickets are delivered to the phone of the lead booker without a QR code. This code is added by stewards outside the stadium using a hand held device which connects to your phone through the app using bluetooth connectivity. The ticket has to be displayed within the app in order for the QR code to be added, which is why a screenshot is no good.
If no one in your party has a compatible phone then you should contact UEFA. You will need to persuade them to allow you to collect paper tickets from the stadium on match day. The experience of members at the Nations League, where this system was used, was that UEFA will take some convincing !