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How can I design separately for desktop, tablet and mobile?

Hey everyone!
I’m having an issue with my project where I try to change something on desktop view appears on my mobile view as well for example the image width and vice versa. Another example is I want 1 picture on my slider on the mobile view and 3 on the desktop view for which I’m having problems with.
Is there any way where we can design separately for desktop, mobile and tablet? And where do I add changes in the settings for the same?

Any help would be highly appreciated!

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