Is the currency unit in AWS Budgets always in USD?

Your default payment currency is the currency your default payment method will be charged in. This is also the currency displayed on your invoice under your default service provider. AWS supports a variety of currencies, depending on the service provider you receive invoices in.

You can check your current currency under Payment Preferences on the Billing dashboard.

https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/manage-payment-method.html

Yes. In the billing and reporting AWS-native tools such as Budgets, Cost Explorer, Cost and Usage Report, the display currency is USD, regardless of the currency that you are charged on your invoices.