How do I know when my veg is ready?

This is my first year planting veg (from seed and from starters) in an outdoors garden…I’ve been primarily an indoors tropical/succulent person, never gardening for food.

So please help me out with a rather silly question: How do I know when my below-ground veggies are ready? I have radishes, fennel, carrots, bunched onions, and I’m unsure as to how I’ll know if they’re big enough, not ripe, or too big.

And how do you even tell your vegetables apart? I’m sure they’ll have different aboveground foliage but I’m not sure if I’m clever enough to tell them all apart. (They’re planted in rings in large round containers but some of the seeds probably got jostled out of line a bit.)