In general, if you want to use data from an Entity, you need to use a lookup expression: https://docs.getodk.org/form-datasets/#lookup-expressions
If you want to autopopulate a value that is hidden or read-only, you can use a calculate
. To autopopulate a value that a user can change, use a dynamic default.
If this doesn't answer your question, please either show a sample form or tell us more about what you're trying to do. Feel free to write in the language that is most comfortable to you!
And when you have a moment, we'd love to learn more about you and what you do in the introduction thread!