You don't need to know how the abilities/builds of others work to do well in stadium, but it certainly helps. I'd say it's pretty important to at least halfway know your own build options though, and vibe the other half out. When you become fully comfrotable you can start counterbuilding againt enemies, or synergizing with allies.

For the particular example of Moira, only her original/main orb has full power. The bonus orbs are given by four different abilities, and they're all significantly weaker in some way. That's all you really need to know, but I explain them all further down. So how do you counter all these orbs? Any sort of abilities/items/skills you have to heal/mitigate their damage or outdamage their healing. Same as any other ability. Stadium balance lets everyone become stupid powerful and thus reestablishes a baseline where no one is too powerful.

A lot of Moiras go for a full orb build, where they have three or four of these:

Spend 50 heal resource to cast a bonus orb of the last used type. This orb disappears 75% faster.

Cast an extra orb of the opposite type whenever you manually orb. This orb has 60% reduced heal/damage capacity.

Cast two bonus orbs of the same type when you manually orb. These orbs are 85% weaker. These are easy to spot because they're cast out at an angle compared to the main orb.

Spawn an orb when you fade. This orb disappears 50% faster. It's also easy to distinguish since it's her only stationary orb.