If you're small, everything will fit nicely in a monorepo.
If you're large, you'll want lots of repos. There aren't really any off the shelf monorepo options that scale super well, so using a bunch of small repos is a great way to deal with the problem. Plus, you probably don't have a full time staff babysitting the source repos, so you want some isolation. If someone in another org is breaking stuff left and right, you don't want the other orgs to be affected.
If you're GIGANTIC, monorepos are a pretty great option again. You'll probably have to build your own and then have a full time group of people maintain it, but that's not a huge problem for you because you're a gigantic tech company. You can set up an elaborate build system that takes advantage of the fact that the entire system is versioned together, which can let you almost completely eliminate version dependency hell. You can customize all of your tools to understand the rules for your new system. It's a huge undertaking, but it pays off because you've got a hundred thousand software engineers.