Start design for mobile or desktop first?
Mobile first is a design philosophy that seeks to begin the design process with the mobile version of a product and then adapt it for larger screens, such as a computer or tablet.
From Google
Design a User Experience for Social Good & Prepare for Jobs: Introduction to Course 7
With mobile-first designs, users can get to content quickly because they have to load fewer elements and features. This is important because mobile phones do not have as much computer power as desktop computers. If you start by designing for a bigger screen, it can be hard to fit all of the design elements of a complicated screen into a smaller screen without slowing down the load time. This is just one more reason why designing for mobile first can be helpful.
Design with the 4 Cs
When you're building a new product, it's important to focus your design process on one device first. Start on the device where your target audience, also known as target users, spends the most time doing tasks that are similar or related to the tasks you're designing for.
Consistency: Having a uniform design, so users can expect the design to feel familiar across devices and products
Continuity: Providing users with a smooth and uninterrupted experience as they move between devices
Context: Designing for the needs of a specific device and the way in which the user will use that device in any given situation
Complementary: Taking into account how the product design on each device can make the overall user experience better