Dynamic forms for SharePoint has been around since 2012. You can find all previous version of the Classic DFFS over at my other site SPJSBlog.com. The Classic DFFS is still being maintained and you can use it for WSS 3.0, SP 2010, SP 2013 and for classic SPO sites.
In this site you find the Modern DFFS version - built from scratct exclusively for the Modern SharePoint experience on Microsoft 365. Please note that it does not support SP 2016 and SP 2019 on premises.
Using the Modern DFFS you can create customized responsive forms for your SharePoint lists and libraries with simple drag-and-drop. Choose from all form fields configured for the list, add custom text or HTML and arrange your form using tabs.
Use the simple and powerful rulebuilder to make your form dynamic based on field value, SharePoint group membership and other triggers.