When adult teeth emerge from your mouth, they do it slowly and considering that you are chewing everyday they get slowly put into place by the teeth you already have. If they were to stand out they would consistently get more pressure on them (by the opposite teeth that hit them when you bite) and that would shape their alignment.

What do you mean by ridges? Grooves and cusps? They have those before erupting, yes. Crowns are fully shaped before erupting, though they can be worn down over time, ideally they would not be.

Fit together how? Within the arch how they fit against their neighbours? or the occlusion between arches? In absence of malocclusions ("bad bites", which are relatively common), teeth fit together well- i.e. cusp and fossa relationship between top and bottom teeth, presumably by some evolutionary process? Though in an individuals mouth as they erupt they tend to move into place until they are stopped by neighbours and adjacent teeth on the opposite arch.

The goal of braces in fixing a bite is generally to put the teeth, as they are formed before erupting, into their ideal positions to fit together well.

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