Yes, I’m using N26 actively myself. They follow Mastercard’s rates, which you can find here: https://www.mastercard.com/global/currencyconversion/
Exact markup compared to interbank varies a bit per currency pair, you can check the spread yourself on the page above. But from my experience, any bank that says 1% currency conversion fee really means 1% in addition to the MC or Visa rates, not the interbank rate…
These are MC’s rates for yesterday:
1 USD in EUR = 0.887705
1 EUR in USD = 1.126200
At these rates you can start with 100 EUR, change them to USD and back and end up with
100 EUR * 1.126200 * 0.887705 = 99.9733371 EUR
That means there’s a currency markup of only 0.01333145% per transaction. Not bad.
And I’ve personally tested that both N26 and Schwab actually give the rates they claim.