What is the difference between friendship and a friendly relationship between you and strangers?

I live in a country (Finland) that consistently has very high levels of social trust and trust in government and other institutions. As a result, I do often feel at ease and confident in the future containing the same necessities for happiness that I have experienced in the past. I also have quite few close friends, to the point that I can see in practice very often the difference in my resilience to pain, sense of security, and enjoyment of pleasure when I am alone and when I am with friends. In my mind, I can separate these two sources of pleasure from social life, but they also feel very similar to me.

So when reading about Epicurus, I can see clearly the unique benefits of strong friendships on lifetime pleasure. One thing that I do not understand though is whether, in Epicurus' time, there was such a thing as "social trust" among communities larger than say, the Garden, and whether this is thought to possibly contribute to a similar sense of security. Is a sense of security drawn from strangers and political institutions less reliable and therefore a false opinion? I have the sense that modern egalitarian democracies that are resilient to corruption have achieved remarkable levels of trust and accountability from institutions and it is possible ancient societies could have been lacking in this department. I don't have the historical knowledge to say this with certainty, however.

Are there bounds to the concept of friendship in the sense thought of by Epicurus (the characteristics of it and the specific benefits it provides) that rule out other forms of kinship with human beings, kinds that do not even necessarily require knowing the other person? Thank you to anyone who understands what is confusing me here and can maybe point to some texts that elucidate the issue for me.