You can open a US dollar account in a non-US bank. Some banks don't offer this service to ordinary private citizens (others do), but I think pretty much all will if you are a (non-sanctioned) business or foreign government or high net worth individual looking to deposit large sums of cash (millions).
The bank needs to somehow hold US dollars to back its US dollar deposits, but there are various ways of doing that – e.g. put the money in its own account(s) in a US bank, hold physical US dollars in a vault somewhere, purchase US treasuries, use the deposit to fund lending denominated in US dollars (huge market for US dollar loans outside the US) – likely it is using some combination of those
So probably some of these Russian assets in Europe are euros, some are US dollars, some are other financial assets such precious metals, stocks and bonds, units in managed funds, etc – and they are just all being converted to euros for reporting purposes
The bank needs to somehow hold US dollars to back its US dollar deposits, but there are various ways of doing that – e.g. put the money in its own account(s) in a US bank, hold physical US dollars in a vault somewhere, purchase US treasuries, use the deposit to fund lending denominated in US dollars (huge market for US dollar loans outside the US) – likely it is using some combination of those
So probably some of these Russian assets in Europe are euros, some are US dollars, some are other financial assets such precious metals, stocks and bonds, units in managed funds, etc – and they are just all being converted to euros for reporting purposes