Which is great to see but other exchanges have probably had a influx of new users signing up and depositing money on their exchange.
Probably, but those figures are still net change. So any new deposits are being cancelled out and more by withdrawals.
The thing I am looking for about the outflows is, is it 500 individuals pulling out their 0.5BTC or 1 major institution puling out 250BTC?
Not sure such data exists, but I suppose you could trawl through some Binance or Coinbase withdrawal transactions and see the general trend. I would imagine it would be a mix of both.
So this 2.25 million Bitcoin: is that the amount found on the blockchain on addresses known to belong to exchanges? Or is it the amount in all user accounts on those exchanges?
It's the amount on the blockchain. If you click on the link on the tweet I shared above, it directs you to the glassnode page where they explain their methodology:
These metrics utilize glassnode's entity-adjustment clustering algorithms, to provide a best estimate of the true balance held by each cohort.
They of course wouldn't be able to have data on how much is in all the accounts on an exchange unless the exchange chose to share those data, and of course no exchange is going to share data which show them to be insolvent.