Basic stuff, but good for newbies.
Think of subreddits as small forums with very specific niches.
Take an example GameDev subreddit -
https://www.reddit.com/r/gamedev/
They have 302K subscribers, all who are interested in game development.
Any time someone posts in this subreddit, and it gets a few upvotes, all the subscribers will see the post in their news feeds.
Got a product related to this? Udemy courses, books about game development, game development software, etc.
I've promoted thousands of products this way, and it works every time. People on Reddit are buyers. Subreddits are huge lists of buyers of specific niches.
Think about it!
Best of luck in your ventures!