Try as hard as I might I cannot find any specifics as to what defines nearly any border in Africa, let alone most of the world. Is there some sort of site that has this information? I'm looking for specifics, like a geographical feature or a border treaty defining parallels.
I merely chose Algeria at random, but any site that explains the reason behind borders would be fantastic. I'm aware of both the 1830 Conquest and the Berlin Conference but I still don't understand why the border is like it is and what legal document actually states that this is what the border is.
Algeria's southern borders are chosen pretty randomly, defined by latitude and longitude. But the border with Morocco follows natural features, mostly the interior Atlas Mountains and various escarpements and watershed boundaries. Several rivers like the Oued Kiss and the Draa form the border in various parts. Algeria's border with Tunisia is more arbitrary in the Sahara, but from the Chott el Djerid it appears to mostly follow watershed boundaries and ridges within the Atlas. So not the best setup, but not horrible either. IMO the Romans did a better job, since the Romans divided Mauretania Tingitana (with most of modern Morocco) from the rest of Mauretania with the Moulouya River (Malva in Roman times), which IIRC is one of the larger rivers of the region. Of course, "Morocco" as it is now does not include many parts historically ruled by Moroccan states. "Algeria" itself is pretty artificial, given the area tended to be divided between states centered in Ifriqiya and states in Morocco since the Roman Empire (Mauretania vs Africa).
A lot of African borders are nowhere near as bad as they're stereotyped to be, given that they often follow geography. The problem is most of those geographical features never divided an area but instead the opposite--they served as a point of unity.
The concept of monoethnic states is actually a pretty Western-centric concept: and even then, it took Europe around a century of violence and ethnic cleansing to achieve that. Traditional African states such as the Buganda Kingdom, Kingdom of Kongo, Sokoto Caliphate, etc. were all ethnically diverse. So while European states definitely didn't care about ethnic divisions when they created borders, the fact that they included multiple ethnicities in the same state is pretty logical in an African context, even if unintentional.
What helped is that Europeans lumped many potential ethnic groups in their own countries under one header. For instance, a peasant from the Tyrol and a peasant from Mecklenburg are both Germans, even if they could hardly understand each other. Some countries got really successful at this, like how France got Gascons and Picards to both become Frenchmen.