Millions of people are searching simultaneously at google.com or youtube.com but servers are not crashing. Issue is not traffic overload but something else.
And similar concepts of maintaining by sysadmins are practiced.
Press and hold the F5 key on your keyboard for 2 minutes while on gmail.com. You will get a "service unavailable" error. About 500 other people whose data happens to be cohosted with you will also get the same error, and all of you will be unable to send or receive email, even by IMAP, for about 10 mins while your particular corner of the data store is restarted and the data integrity checked.
That doesn't happen on Google.com
Don't believe me how slow it is on a regular instance? Try going to "All mail", selecting all of your emails, and applying a label to them all. In my experience, it can only label about 50 mails per second, so it can take hours to do them all. It will keep going if you quit the browser, but will stop if the gmail devs do a software update, which they seem to do on usually tuesdays, but never fridays or the weekends.
I’m sure they have machines that are only used to serve G Suite and Google One customers, and maybe some other VIPs, but regular heavy users? It sounds like an urban legend to me.
for Gmail, you have to log in. It has to know it is you. It has to know that everything it is serving you is only for you. It has to retain copies of documents that -- as far as it knows -- are unique to you or whomever you share with. It has to do all this while keeping that information safe from other people who might want to take a look at it.
And that's just the items I can think of in real time while typing.