Can I use handlebars in Content Builder to pull personalization data from multiple data extensions?
Example: {{Contacts.Attributes.DataExtension.Field}}
Or does this only work on the backend working with JSON.
You can use it with Guided Template Language. If you just want to access personalisation within the send data extension, eg first name, you could use {{first name}}
%%[ VAR @var, @this SET @this = [Subscriber_Key] SET @var = Lookup ("leads", "favorite_car", "Subscriber_Key, @this) ]%%