There are 3 ways to make a website mobile-friendly: responsive web design, dynamic serving, and separate urls.
Separate urls: basically you have siteexample.com for desktop, and m.siteexample.com for mobile. One of the downsides is getting links; you'll either get a link to your desktop site or your mobile site, so you're splitting link juice between the two.
Dynamic serving: Uses the same URL regardless of device, but generates a different version of HTML for different device types based on what the server knows about the user’s browser. This can be technically difficult, and you basically need to build/maintain 2 sites.
Responsive web design: this is the gold standard. You design your site so that it looks differently on different screen sizes.
That said, there's more to making a site mobile-friendly than just being responsive. You have to make sure the text is big enough on mobile devices and the calls to action are clear. You have to make sure your page loads quickly, and probably take advantage of things like Accelerated Mobile Pages (AMP) to do it.
I actually just published a free guide to mobile SEO, which includes a chapter on responsive web design. It's pretty in-depth and goes into more detail than I can in a forum post. You can check it out here: https://www.coloradoseopros.com/ultimate-guide-mobile-seo/