LeetCode’s Premium service is a popular service that lists most frequently asked questions per company.
Some of LeetCode’s companies for their frequently aske questions section
Blind is a social app where individuals anonymously talk about workplaces. Blind is notorious for users who post fake content, degrading commentary, and other malicious content. However, if users can navigate around the unwanted posts, they can find some very important content regarding companies for which they are interviewing.
Anonymous Professional Network - Blind (teamblind.com)
1point3 acres is an app on a whole other level when it comes to discussing interview material. However, the application is almost entirely in Chinese.
Historically, many non-Chinese users, especially from India, attempted to translate the content. In response, due to political differences, many Chinese users now deliberately write their content in formats that are difficult for translating tools to interpret.
Grokking the Coding Interview by Educative.io is a pretty decent tutorial to get people familiarized with distributed system design.
https://www.educative.io/courses/grokking-the-coding-interview