How is Craft able to reuse fields in different Sections? (As opposed to EE.)
In Craft, all fields are stored in the craft_content table. They are otherwise independent of any field groups, layouts, sections, or anything else.

PRO: This means that any element (Entry, Category, Asset, etc) can access any field.

CON: This means that every element carries the weight of every field. Though if those fields are all empty, the performance impact is almost non-existent.

There's historically been some minor debates over whether or not this tradeoff is worth it... But I think it's safe to say that the benefits vastly outweigh any negligible performance issues. The simple fact is, having all fields in the same table is rarely (never?) the true bottleneck.