What is the main use of Liquid in Shopify themes?

Liquid is a templating language created by Shopify that is primarily used to dynamically generate HTML content in Shopify themes. It allows developers to create flexible and customizable themes by incorporating dynamic data from the Shopify platform, such as product details, collections, and customer information.

When Liquid syntax is utilized, it can include logic, looping, and variable output, which enables developers to adjust the content based on various conditions or inputs from the store. This is crucial for creating personalized shopping experiences, such as displaying different products for different customers or altering content based on inventory levels.

The other options present functionalities that are not within the scope of Liquid. For instance, while online payment processing is vital for e-commerce, it is managed through Shopify's backend systems rather than in the theme layer. User session management also falls under the purview of Shopify’s infrastructure, ensuring secure handling of customer sessions without needing to involve Liquid. Similarly, creating database connections is managed internally by Shopify, abstracting developers from directly interacting with the database, thus allowing them to focus on the presentation layer with Liquid.