Seeing current flow through switches isn't the same as seeing specifically where your power is coming from.

Like, if power from several different sources flows into a single substation before it goes to your home, you know exactly which source created the electricity you're using?

And if I purchase power from a 100% renewable company, is it always only electricity from their generators, never ever mixed in with power from other grid sources during downtime?

No, it's just best estimates based on generation, consumption and flow.

That's like taking a bucket of water out of the ocean and saying you know where it came from. Sure, it might have come from the nearest estuary, but odds are it got mixed in with water that originated somewhere else.

Electricity transmission is a bit like water pressure and distribution. You're not going to get the same electrons (or water molecules) that you subscribed to from various producers in the distribution network put in, the flow causality and the physical embodiments of production are different though related things.

Kirchoff's laws applies though. It should be possible to calculate where the power in your outlet comes from for the power line operators.