How do people advertise their game except for buying Ads and Sponsors?
You can try games like GRP (Plaza Connect - Roblox). You can advertise on YouTube, Twitter etc. Meet random people and tell them about your game/group.
There are many ways to advertise without spending money. I must say though, the Roblox Ads and sponsors are VERY effective and I recommend spending money on this platform.
I recommend you go to social media. Also, spread the word with your friends for them to help out and get people to play. Also as @lodebt mentioned GRP is a great place to go for advertising.
Many discord servers have communities that are quite open to new games, also you can make youtube videos on the game, you can make twitter posts, etc.
you could try advertising it on some discord servers as long as they have a advertising channel