The main screen of Koala Wallet shows your wallet's total US dollar value, and individual asset balances and values below it. If instead of the US dollar you prefer to use a different currency, you can easily change it.
First press the Apps button at the bottom of the main screen.
At the top right hand corner of the screen, tap the Settings ⚙️ button
Then go to "Currency". There you can choose fiat currencies from around the world.
Your final balance and asset values will immediately update according to your selection. When sending a transaction, the equivalent fiat value displayed will also use the new currency.
The asset prices can be a combination of data from CoinGecko.com and other services like DEXs. Coingecko uses an average of many different services and exchanges. Prices can be volatile and you should be mindful of possible price discrepancies when sending a transaction by defining a fiat value.