You can't because there is a network monopoly and barrier to entry is sky high. Realistically, your only option is to piggyback on Android by developing an Android-compatible OS like Huawei did. However this will soon become impossible anyway, as Google will abuse Play Integrity to make your device unusable.

The only way out is either regulation or a whole paradigm shift that renders phones irrelevant. I'm not sure the latter will happen any time soon.