So the money does not leave, new owners come into the country and control its flow?
Most reserves (which are needed for interbank settlement) are accounting entries at a central bank these days, and only domestically/regionally licensed banks are usually allowed to hold central bank accounts.
One big exception are foreign-controlled subsidiaries of large foreign banks, but even these are subject to local laws and regulations, in particular sanctions enforcement.
When you hold a foreign currency account at your regional bank, this amounts to a claim in the amount of your balance against them, which is only indirectly backed by your bank‘s claim against its correspondent in the country where your currency‘s central bank is located.