Unless you have been hiding under a rock you will see the dominance of mobile usage today. Everyone is on their phones (way too much). But often website design agencies forget this and do a desktop top design approach.
Mobile first with desktop in parallel is the best approach for an effective design and it has an important factor for Google and SEO. Since 202 Google has focused on mobile first indexing which means in layperson terms mobile is checked first by the search engine
Mobile-friendliness is crucial for SEO because more people are using mobile devices to access the internet.
In Search Console, you can easily check if your site uses mobile-first indexing. Look at the settings page for the status. You can also use the URL Inspection Tool to check a specific URL and see how it was last crawled.