Products. Each one gets a photo, a description, a price and any options, such as size or colour.
The shop page. Products laid out in a grid. Customers browse, filter, and open anything for details.
The cart. Several items, one checkout, with the cart following the customer across the site.
Payments. A card processor connected, so customers pay by card or wallet and the money goes to your bank.
Shipping or pickup. Flat, calculated or free shipping, or local pickup, chosen at checkout.
Orders. Each one lands in the dashboard with the customer, the items and the payment status.
A Shopify or other third-party cart can be bolted onto an existing site. That is two systems, two bills, and a shop that often looks like a different website.