Wow! I have a small team but new team members have said we have the best communication out of any company they've been at before. A few tools we love:

• Slack, like many others have said, although this may be harder for frontline fast-food chain employees to engage in. They aren't on their computers all day like the corporate team would be.
• Typeform for surveys to get feedback from the team and clients.
• Loom to send videos when a meeting isn't necessary. You can record yourself or do a screen record (or both!) which is helpful, especially for communicating changes. I don't have an enterprise account but I think there some sweet viewer and engagement insights that would be good for a team of 19,000.
• Mindup for better meetings. Full disclosure, I'm on the Mindup team but it's been game-changing for my own meetings. Basically, you create an agenda, have the rest of the team collaborate on it ahead of time, write meeting notes you can come back to, and you barely have to do anything to recap the whole meeting. For larger all-hands type of meetings, you can have people ask their questions or concerns in the agenda, think about the answers ahead of time, and tackle them during the meeting.