Well, you often have no other choice. You can go out of your way and install an open source OS, but then there might still be a backdoor in your hardware. Ultimatively, this can not be solved technically, but socially. In a country with a strong rule of law and democracy, you
should be able to trust that the state builds no backdoors into your devices when they say they don't. And you should be able to trust the manufacturers that they don't do it by themselves. But unfortunately they have incentives to go both ways (short term profit + appeasing governments on the one hand, vs. gaining consumer trust on the other).
I'll go meta here: How can we trust food companies? They have incentives to decieve us, to adulterate their products, produce as cheaply as possible and sell as expensively as possible. There are magazines, websites, that do nothing but test food. It blows my mind when I think about that. How antagonistic is our society, when the people who make our food are working against us. The solution we developed for that are checks and balances in form of journalism and consumer protection laws. Sometimes it works, sometimes not. Whether its adulterants in food or backdoors in software, it's a similar problem.