Every campaign shoudl have its own import rule configured, where you mention the location on logger and file name from which it has to import the contacts. Contacts gets imported when file is available at location mentioned in import rule. However these will not be provided to dialer until it matches the configured time mentioned under the campaign setting.
To answer your question, contact will be uploaded to respective dialer for which you want to start import and dialing contacts. You can customize the settings under import rule and campaign manager to match your requirement.