What makes something a "doctrine" of the Orthodox church, beyond the early ecumenical councils?

Sorry if I'm a bit wordy, I'm a bit new to apologetics and am trying to understand church history. This is something I struggle to understand a bit. With Catholicism, you have the Pope as a unifying force and a lot of ecumenical councils that lay out the rules of infallibility. So it's usually easy to determine if someone agrees with "Catholic theology" or not. With Lutherans and other Protestant groups, the rules are less strict, and they tend to see "the church" as more universal. But then there's Orthodox, a group that believes they are the one true church and even in some form of "no salvation outside the church." But how does the Orthodox church decide which teachings one must accept to be Orthodox? Oriental Orthodoxy accepts three councils as infallible, and I think most Orthodox accept seven. But I don't really see explicit Orthodox doctrine on which parts of these councils are infallible the way that later Catholic councils have. (The canons of the councils themselves that I've read don't use the word "infallible", and Lutherans see the councils as authoritative but below scripture.) Orthodoxy has a specific view of the trinity and a specific view on salvation that differ from Lutheranism, despite both groups generally agreeing with those councils. **So what it it that determines these Orthodox beliefs, if they don't consider anything outside 3-7 ecumenical councils and scripture to be infallible?** If Orthodox consider their view of the trinity to be fallible, why? Doesn't the Catholic view also follow the Nicene creed? As an application of my question, how does the Orthodox church decide what the "one true church" is when making statements like "no salvation outside the church"? If Oriental Orthodoxy and Eastern Orthodoxy are the same church, despite doctrinal disagreements, then what makes Eastern Orthodoxy and Lutheranism not the same church, despite many similarities? In the early church, weren't there different congregations with bishops that were considered the same church, despite having different practices? Where's the line, according to Orthodoxy?