Are there general stats to shoot for on relics? I’m guessing most people go crit for primary where possible and attack % for secondary? Maybe speed for primary on boots, or energy regen for support characters?
You should read the character’s skill, ultimate, talent, etc. descriptions to see which stat that particular ability scales off of.
For example, March 7th’s skill and talent scale off of her def, so you care about that stat.
It take depends on the character’s role. DPS characters, for example, will shoot for crit rate or damage on body: which one is better depends on the character (e.g. Yanqing probably doesn’t want crit rate). For boots, ATK is often preferred over SPD, because if the DPS is faster than your supports, you may run out of skill points. For the rope, energy regen is usually best because it’s the only possible source, but ATK is fine too.
Check these sites for builds. They list preferred main and substats for each character and slot.