Mobile site vs desktop

Hello I’ve recently been helping a client setup her blog, she uses Elementor to create and edit content on the blog. I’ve been running into some issues with the mobile version of the site, specifically content being served to mobile devices.

For an example, I’ll write some css to target classes that apply to all blog posts to say hide a post summary. The CSS works as intended on the desktop version of the site no matter the viewport width, but when viewing the site on a smartphone, it’s as if the CSS was never written. Another issue that I’ve ran into is publishing content on the desktop and it takes 6+ hours for the content to load on mobile if at all.

Does elementor serve one version of a website to desktop users and then another version to mobile users? Or does it say apply different classes to elements when viewed on a mobile device?

Thank you for your assistance.