I very much doubt it was used in this case, but https://www.websequencediagrams.com/ is great if you want to do Message Sequence Charts.

The blues look like a PowerPoint theme. You might like http://blockdiag.com/en/seqdiag/index.html

There are many ways to go about, but it could be easily achieved with any diagramming or graphics application. For example, LibreOffice Draw or Microsoft Visio.

No idea, but if you want to make similar diagrams I recommend PlantUML. Default colours suck a bit but they are easy to change.

My first guess is Visio. (Followed by Excel, PowerPoint, or Word.)