You can’t have access to Facebook’s newsfeed algorithm or Google search source code. However, you can get pretty close to learning what’s under their hood by researching which technologies are using.
Most tech companies are very generous in that regard, even open-sourcing pieces of their platforms. For example, you can go to Grammarly’s engineering blog and easily spot that they are using NPL/ML. Or you could go to Netflix’s Tech Blog and see how Data is a clear theme among their posts.
Another good way to learn about other companies’ tech stacks is to visit their career sites. It is effortless to reverse engineer a system architecture if they ask engineers with experience in AWS, Kubernetes, and DynamoDB.
Most of the big tech companies share a similar tech stack. It will give you a baseline of technologies you should consider. But it would be best if you didn’t stop there. Small startups win because they have an advantage, and most of the time is not in the form of industry knowledge but because they use new technologies that allow them to move faster.