What application would you recommend for building an internal community within your organization where subject matter experts can share ideas and create efficiencies and engage with one and other. For example is Yammer a good choice?

In terms of plateforms, several tools exist but to manage content no better than a core team of dedicated and trusted people. If tool only and to allow people to keep in touch my company uses Facebook tool workplace and work chat along with teams. 

In terms of tools there's plenty, Slack, MS Teams for comms, then Confluence for docs or even Git as source of information I believe more important is how to use those tools. Keeping open communication and conducting Webinars or internal open talks about technology can be a good driver. When something has to be written somewhere, start by adopting RFCs to gather insights from all stakeholders. 
Is also crucial to conduct recurrent tech talks with structured topics that have follow up sessions on-demand for specific agenda items. 

We use Teams channels for groups like our Excel Users Group where people can pose questions, lessons learned and questions to the broader audience. We also like being able to save documents to that channel for people to reference as needed. I had never heard of Yammer but I plan to look into it more now that I have seen it suggested quite a few times in these responses.

Avoid adding another platform that requires their attention. Best case you can include it in something already established, like Yammer works well with Sharepoint and Office, and MS Teams as well. I think Microsoft is really onto something when it comes to internal communities and would be my go-to choice as well. External communities, a complete different horse, though.

MS Viva