No. They use ADP, Concur, WordPress (check out AWS official blogs), Salesforce and third party SaaS products just like everyone else.
Yeah, they know the "keep everything default as much as you can" rule better than anyone else with SaaS providers. What a load of gobshite.
Are you saying they should keep everything in house? I can tell you from personal experience that the external facing services from one of the major cloud providers handle spikes in traffic and has much better uptime than many internal systems.
Nope. Microsoft (for example) use SAP. It would be crazy for them to spend millions building something similar to SAP.