But to build complex web applications we would need complex frameworks and libraries such as Angular, React.js, Vue.js etc.
There are certain frameworks which target specific devices such as mobile/tablets which includes React Native, Cordova, Ionic, Framework7, Xamarin, Flutter etc.
Its basically part of the application/website which you cannot see as its mostly to do with server side processing and storage of data.
Some of the common server side languages and frameworks which is used are php, java, python, node.js, C# using .Net Core, laravel, slim etc.
These are basically used to write apis to process the data and interact with database such as Mysql, mongodb, Ms sql server, PostgreSQL, Oracle etc.
CMS & eCommerce
CMS (content management system) is a software esp targetting the web which is used to create/manage contents in a website. Offcourse the CMS needs to be customized esp in terms of GUI and structure to suite customers requirement. Hence if there is a change in terms of contents/media you dont have to modify the code but instead just modify/upload it from the admin interface provided by CMS. WordPress is a very popular open source CMS and its market share is like 80-90% when content based website is concerned.
There are ecommerce platforms which help you setup a web shop for you & sell your products. Shopify, woocommerce built over wordpress are commerce platform which is very popular. Magento and prestashop and numerous other platforms are popular too. Please keep in mind a CMS or a web shop can be custom built in certain cases.
Please do not hesitate to share your idea
Just discuss your requirements to take it further and make it a reality