External (off site) optimization is optimizing external content. In fact, Google My Business is included below.
Important here is the weight of the page where the backlink comes from. A link from a valuable domain, such as Facebook, Wikipedia, ... weighs heavier than a website with little relevance and visitors. Note that for example Facebook is a very important domain with a very high value but that the pages within Facebook also have their own value.
A possible piste to collect backlinks are the so-called company guides . The most famous regional company guides are Yellow Pages, Infobel and Yelp.
The most obvious location for backlinks is social media. A page on Facebook and LinkedIn strengthens the online presence.
Of course, there are nicer things to do than creating accounts on these pages, but all of them help. The more diverse the cake is, the better. See the chart below with the main local ranking factors of Moz (based on a survey among SEO specialists):
This is obviously only the tip of the iceberg. Search engine optimization is a time-consuming and recurring process. To get links from outside you will also have to have enough interesting content on the website. This can be done by writing articles yourself or by publishing the articles that the group offers, which is a great asset.