If we are talking about aesthetical beauty of the sites, the underlying tech doesn't matter at all, it's all about the CSS.
Otherwise, Flask takes care mostly of backend APIs and uses Jinja as a template engine. React mainly handles the rendering of the app state into the DOM. You can still use Flask for the backend APIs and use React in the frontend, if that's the route you want to go.
But, as I've said, it doesn't really matter. Plenty of successful web apps are built with Flask, plenty of other successful web apps are built with React, and some of them are built with both. You should pursue what you are interested in.
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If we are talking about aesthetical beauty of the sites, the underlying tech doesn't matter at all, it's all about the CSS.
Otherwise, Flask takes care mostly of backend APIs and uses Jinja as a template engine. React mainly handles the rendering of the app state into the DOM. You can still use Flask for the backend APIs and use React in the frontend, if that's the route you want to go.
But, as I've said, it doesn't really matter. Plenty of successful web apps are built with Flask, plenty of other successful web apps are built with React, and some of them are built with both. You should pursue what you are interested in.