You can only see messages, files and resources that you have access to.
View and edit permissions for each project or team resource are managed in the individual integration (e.g. Slack, Confluence, Box, etc.).
If you don’t have access to those resources, you’ll be able to see them on the search results, but you’ll need to request access in order to view, comment on, or edit them.
Don’t worry — you (or your coworkers) won’t be able to find or access files that are stored in personal folders or Slack channels.