Comment by greysteil

Through the API - in particular the info endpoint[1], combined with the fact that Reddit IDs are base36 encoded sequentially increasing integers[2]. You can get 100 objects at a time, so if you make ~3 requests a second it's enough to get all of the new posts and comments.

[1] https://www.reddit.com/dev/api/#GET_api_info

[2] https://www.reddit.com/dev/api/#fullnames