Mendix Studio is only running inside your browser. It’s an online service. If you want to work offline you need to download studio pro.
Adding to Andreas Blaesius’s answer: To open the Mendix Studio in your browser, on the Mendix platform – Apps (you can click “Apps” in the navigation in the top of your page here: https://sprintr.home.mendix.com/index.html), go to your application and click Edit App (top right). If you get a page saying that “No Development Line Is Selected” then you can refer to this link under “Enabling Studio for a Development Line”: https://docs.mendix.com/refguide/collaborative-development