Do you need to buy an Apple device to watch their streaming? I have no idea but I do have Apple Music on Android.
It seems very unlikely that, if Apple were to own the broadcast rights to F1, they would be able to restrict access to only users of their own devices. The competition authorities would go mental if what is literally the world's richest company were to attempt a vertical monopoly. It's one thing to own the broadcast rights - somebody has to own them - and charge for that, but another to undermine your competition in the production and sale of physical devices or software servies by limiting access to something that has characteristics of a unique public good.
If Liberty are thinking of selling to Apple, they would also (one expects) have to consider the effect on the teams. Many of the bigger-money sponsors on the current grid are direct competitors of Apple's. They might not particularly mind if the races are accessible through an Apple service, but if Apple were to exploit an exclusive right to broadcast as a means of forcing customers away from Dell, Microsoft, Google, Hewlett Packard, et al., the sponsors would not be pleased. The teams do not have a vote on the matter, only Liberty does, but their voices would be heard.