How do I make a program that works offline?

Keep the data locally on the computer if the program runs on one machine and you want it to work with no internet, accepting that backups are your job. Choose the cloud if several computers or branches need to see the same records at the same moment, and you accept that work requires an internet connection and a sign-in. The deciding question is how many computers need the data.

Take a copy of the program's data regularly and keep it somewhere other than the computer itself: an external drive, a second disk or cloud storage. A simple rule is one copy every working day and a weekly copy kept outside the shop. Most important, test a restore at least once, because a backup you have never restored cannot be trusted when you need it.

Yes, provided the data is stored in one place every computer can reach, which is usually a cloud database that each machine signs in to over the internet. Installing the same program on several computers with local data does not connect them: each machine keeps its own records. So decide when you build, based on how many computers need the same records.

A cashier program needs four things: a product list with prices, a fast sales screen that adds items and works out tax and totals, a printed receipt, and a report of the day's sales. You can buy a ready-made package, hire a developer, or describe what you want to an AI builder that builds it around your shop. Start with selling and receipts, then add stock and offers later.