For every mod that goes into your server, it has to be run client-side. However, playing with mods is not as simple as just uploading them to your server. Having a modded Minecraft server is a great way to customize your gameplay experience.
Understanding Plugin Installation Addon.How to Prevent X-Ray on a Minecraft Server.How to Convert a Bedrock World to Java Edition.How to add mods to an existing modpack (client-side).
