Where players actually look
- Server lists: for Minecraft, sites such as minecraft-server-list.com, TopG, Planet Minecraft's server section and minecraftservers.org, each with a vote system. One good listing on each, voted on by your regulars, brings steady traffic. Rust has the in-game browser plus rust-servers.net and similar lists.
- Discord: your own server first; then "server advertising" channels of larger communities that explicitly allow it. Post once, well; never DM people.
- Reddit: r/mcservers (Minecraft) allows one post per server per week with a fixed format. Other subreddits mostly ban advertising, so read the rules or you get banned along with the post.
- YouTube / TikTok: a 60-second tour of your spawn and what makes the server different. Even a hundred views from the right people are worth more than a thousand spam posts.
What a listing needs
Name, one-line pitch, the version and edition, the rules that matter (PvP? claims? whitelist?), the address, a banner (468×60 for most lists), your Discord. Update it when things change; a listing that says "Season 2" when you are on season 5 looks dead.
What does not work
Copy-pasting your IP into every Discord you can find gets you banned and marks your server as spam. Buying "players" gets bots. Paying for votes gets delisted.