How can Agile methodologies, CI/CD be applied to massive, monolithic software projects with 100s/1000s of developers?
Well, you won't be able to apply an Agile methodology to a large team. One of usual principle is to work on Pizza Team (Less than 10 persons which can share a large pizza for diner together) because the lack of formalism advocated by Agile process makes it hardly applicable to a large team, the information would be scattered and some would be lost in the crowd.

So before thinking about the CI system, you'll have to review the organization around the software you build.
This mainly need to refactor a large project in smaller independent pieces, this could be separate softwares (and you'll take the path of microservices) or it could just be an architecture design within the software, where a class/object/library is given to a small team. This team has to document its service input and output (service contract/service orientation).

Once this is done you can split the responsibility of each independent piece to a smaller team which will be able to apply an Agile process.
This may solve the CI/CD problem, splitting the code base would allow each team to follow it's own pace, as long as it's interface is properly versioned and that the input/output are fixed for a type of call you can make parts of the product evolve while some other part keep a previous behavior.

The main drawback is that the service contracts from each part must be compatible between versions in use, which need a dependency system to know which part use which version of what before dropping an unused version of an API. This brings problems to make breaking changes and usually need a careful transverse planning between producer and consumers.