Unlike Compound where the reserves are denominated in the underlying, Euler's reserves are stored in the internal bookkeeping units that represent EToken balances. This means that they accrue interest over time, as with any other EToken deposit. Of course, Compound governance could periodically choose to withdraw their reserves and re-deposit them in the pool to earn this interest, but in Euler it happens automatically and continuously. Similar to Euler, AAVE deposits earned reserve interest into a special treasury account that owns the aTokens, however this is much less efficient than the special-cased reserves model of Compound/Euler, involving several cross-contract calls. In Euler, the reserves overhead is primarily two SSTORE operations, to slots that would be written to anyway.