There is more to it than being long one currency and short another.
As I posted here How does purchasing your own country's currency as part of a currency pair work?
When you "buy" the USD/JPY, you're not converting your dollars to dollars. Even when you "buy" EUR/JPY, you're not converting dollars to euros.... what you're doing is borrowing yen and lending euros.
Rollover is the interest paid or earned for holding a currency spot position overnight. Each currency has an overnight interbank interest rate associated with it, and because forex is traded in pairs, every trade involves not only 2 different currencies but also two different interest rates. However, unlike what many traders think, foreign exchange rolls are not based on central bank rates. Instead, forex rolls are constructed using forward points which are mostly based on overnight interest rates at which banks borrow unsecured funds from other banks. After all, the foreign exchange market works over-the-counter. Market and spot trades need to be settled and rolled forward every day. If the interest rate on the currency you bought is higher than the interest rate of the currency you sold, you will earn a positive roll. If the interest rate on the currency you bought is lower than the interest rate on the currency you sold, then you will pay rollover.
Since interest is charged and paid, you don't buy or sell anything... you borrow and lend and the currency you funded your account with is the collateral.
If you're simply converting one currency to another, then why are you paid interest to hold a currency? Does the fed mail you an interest check for holding US dollars in your wallet? The only way you get paid interest is by lending the currency out.
So it may be more correct to say when you "buy" the USD/JPY, you've borrowed yen and lent dollars. More precisely, you've borrowed yen from an interbank, sold them for dollars, then lent the dollars to an interbank for an interest payment. The dollars in your account never move. Its just the collateral. To "buy" $1000 of USD/JPY, you need only have $20 in your account. $30 for the EUR/JPY. Why bother to convert $30 on such margin?
If you really wanted to convert your dollars to euros, you'd have to go to europe or find a bank that can make the exchange, or a private party willing to swap. Since you've taken possession of euros and haven't lent them out, you won't get paid an interest rate for holding the euros. You also won't get charged an interest rate for borrowing dollars, since you didn't borrow them, you owned them outright in the beginning.
Of course, if you wanted to find a US bank to loan you dollars, then fly to europe and convert your dollars to euros, then find someone looking to borrow euros,,, you could accomplish the same thing a forex account does by simply buying EUR/USD. In the meantime, you have nothing in your hand but an obligation to pay interest to the US bank and the expectation of an interest payment from the person who borrowed the euros from you. To close the trade, you'd have to demand full payment from the one who borrowed your euros, convert back to dollars, then pay the loan at the US bank.
If you buy AUD/JPY, you've borrowed yen and will be charged an interest rate. You've lent out AUD and will receive an interest rate. You don't hold either one of them. The margin missing from your account is the collateral. Even though you don't hold either currency, you get paid (or charged) the difference in the exchange rate when the trade reverses. The amount you get paid per pip is figured in the "pip cost" or "pip value" which is listed on the quote screen of the forex trading app. If your account is funded with USD and you buy 10k of EUR/USD, you get paid $1 per pip. If you buy EUR/GBP, you get paid $1.60 per pip because your account is in USD, not GBP.