In 2012 while working at eFront, I was in charge of the development of some Microsoft Office addin projects. At that time, we leveraged the COM/VSTO technologies to craft these extensions. The next year, I heard about this new generation...

.NET azure azure static web apps C# react javascript office-addins asp.net

EDIT: this approach is outdated. You can have a look at the sample we released and especially in the README section we compare the different approaches https://github.com/Keluro/Office365-AddinWeb-SignInSample

HTML5 javascript OAUTH office365 office-addins