Ideally, the Combat Mechanics are Derivative of the Dice mechanics... but is rarely the case. "modern" game systems have the tools and reference material to predict probability variance. Thus they can create the modifiers that will make the Core mechanic work in various circumstances, taking into account the kind of probability distribution of each circumstance.
Older and immature systems, have not taken into consideration the subtly of the "math" and have used "stop-gap" measures, breaking away from the core mechanic and introducing all sorts of resultion systems.
example. 2nd ed DnD, where we make Ability Checks, rolling 1d20 on or below the ability score. Why don't we use the same core mechanic when rolling attacks? Or IS the core mechanic the THAC0, where we roll a d20 modified by AC where a target number is provided by a table. GURPS has 3d and roll below a score. The context of the score and the modifiers change, but the mechanic stays, mostly, the same. If GURPS used a Wound Save system, and a Social Condition Save system then TRULY gurps uses one core mechanic no matter what the circumstance is.World Physics and Character Physics. After combat, the Core mechanics are typically expanded to include various kinds of interactions. These interactions are two sets of "Physics" Character and World.
Example. in 2nd ed DnD there was a proficiency check, then a percentile ability check, then a d6 racial ability check (dwarf and elf). Special rules for things that could have been done by the same dice mechanic. In 3rd to 4th ed DND, we have Hit Points, Fort and Fatigue Saves, Spell Points, and Endurance Skill.
Example 2. In GURPS we have the default: Human. GURPS has rules what defines, limits, and abilities of a human. Everything else that is different and exceptions, are then tackled and made into some set of rules. Recently, the heuristics of GURPS have changed much since 4E and the advent of the Information Age. Many of the books have expounded and clarified based on Human performance heuristics.Not a lot of Systems deal with Economics, but World Building economics is the most important, if not the cornerstone, of a sound World Building system. Strangely, Economics has Game Theory making several synergistic relationships with RPGs and Gaming.
Few systems deal with Economics: GURPS and Traveller have very extensive studies into economics for Low Tech and their Merchant Campaigns. DnD DMG 3.0-3.5 has City Building rules, and DnD 3.5 has some extensive world building tools. Although, to my memory of something I've read about 8 years ago I can't recall any in depth economics.
Medieval Demographics Made Easy by John Ross
Town and Fief by Lisa J. Steele
Grain to Gold by John Josten