Is SEO A Two Part Process?

So recently I've had some success with my SEO clients as many of them are receiving positive ranking improvements and some even appear on the first page; however, I've started to see many marketing companies pop up that only focus on one side of SEO. That means they only do backlinking or they only touch up the internal parts of the website. Now I've always been a big believer of packaging the two together as my SEO process is a two part process with both internal and external work. I even include GMB in my SEO work despite it being a different services. Am I missing something or doing something wrong by including all of these in my SEO services?

The way I've always looked at it was a website that has internal SEO with no external SEO is like putting a beautiful new building in the middle of nowhere, or vice versa where you have all external SEO and no internal SEO is like the building being put in the middle of a busy area but the inside is nonexistent or terrible and nobody wants to enter. But that's just how I've seen it and maybe I'm wrong which is why I wanted to hear other peoples opinions.