There is a short intake period after booking before most privileges are activated. The exact timeline varies by facility, but at a typical county jail like Smith County you are generally looking at a few days to about a week before visitation and commissary access open up. During that window, the inmate is being processed, classified, and assigned to their housing unit.
Mail is the exception. InmateAid's letter and postcard service can reach a newly booked inmate even during intake, because incoming mail goes through the mailroom regardless of where someone is in the booking process. Sending a letter right away is a good move. Include your phone number and any other contact information so your friend has it the moment they have access to a phone.
For commissary, your friend will need to be fully processed and assigned before they can place an order. You can deposit money into their account ahead of that, and it will be waiting when commissary access activates.
Call Smith County Jail directly to confirm the specific timeline for visitation approval, since getting on the approved visitor list requires submitting information and going through a background check, and that process should start as soon as possible.