Article 250 is the one that confuses everybody, including a lot of guys who’ve been in the trade for years. The fundamental distinction: grounding is connecting to the earth (the ground rod, the water pipe, the building steel - the electrode system). Bonding is connecting metal parts together so they’re at the same potential. Both matter, but for different reasons.
This is the one that gets misunderstood most often: the ground rod alone is NOT your fault clearing path. Dirt has too much resistance. If a hot wire touches a metal enclosure and the only path back is through a ground rod, the current might not be high enough to trip the breaker - and you’ve got an energized enclosure. The equipment grounding conductor (the green wire or bare copper) is your fault clearing path back through the panel. The ground rod is supplemental - it’s there for lightning and static, not for clearing faults.
Sizing the EGC: Table 250.122. It’s based on the OCPD size, not the load. A 200A feeder needs a 6 AWG copper EGC minimum. A 400A feeder needs a 3 AWG. Don’t size it by load - size it by the breaker. And if you upsize the conductors for voltage drop, you typically need to upsize the EGC proportionally.