Google uses mobile-first indexing since 2020. If your site does not work on mobile, you are invisible to Google. Period.
1. Responsive Design Is Mandatory
Your site must look good on phones, tablets, and desktops. Use Google's Mobile-Friendly Test tool to check.
2. Page Speed Matters on Mobile
53% of mobile visitors leave a site that takes over 3 seconds to load. Compress images. Minimize code. Use a CDN.
3. Content Must Be Readable
Short paragraphs. Subheadings every 2-3 paragraphs. Font size 16px minimum. No horizontal scrolling.