If you want to modify your server settings whether you're on vanilla or any modded version, you can do so in server.properties. Here's how to edit these settings as well as a list of all the settings and what they do.
1. Open the File Manager

2. Navigate to /minecraft/
3. Right click server.properties and click Edit.

4. Here, you can edit the settings to your liking.
5. When finished, save the file and restart the server.

All Server Settings
| Setting | Description |
|---|---|
| allow-flight | true - Allows flight if a fly mod is installed. false - Disallows flying, players in the air for over 5 seconds will be kicked |
| allow-nether | false - Nether portals are disabled. true - Nether portals will send players to the nether normally |
| broadcast-console-to-ops | Sends the console command outputs to all the operators in game |
| broadcast-rcon-to-ops | Sends the RCON console command outputs to all the operators in game |
| difficulty | Peaceful - No mobs Easy - Less and easier mobs Normal - Regular amount of mobs and damage Hard - More armored mobs, more mobs |
| enable-command-block | true - Command blocks are enabled. false - Command blocks are disabled |
| enable-jmx-monitoring | Will show tick times in milliseconds used for debugging and getting more server info |
| enable-rcon | Enables remote access to the server (Remote Console) |
| enable-status | Makes the server appear online in the server browser. If disabled, it will show offline but still be connectable |
| enable-query | Enables GameSpy4 to get information about server from 3rd party programs |
| enforce-secure-profile | If true, players with no Mojang account will not be able to connect to the server |
| enforce-whitelist | Enforces the whitelist to kick players who are not on the server whitelist if enabled |
| entity-broadcast-range-percentage | Controls how close entities need to be for them to appear to players. A higher value means farther away, which means more lag |
| force-gamemode | Players who join will be set to the game mode of the server, even if they gave themselves creative mode previously |
| function-permission-level | Sets the default permission level for functions |
| gamemode | The gamemode of the server. Either survival, creative, adventure, or spectator |
| generate-structures | Defines whether structures like Jungle Temples, Sand Temples and Villages will be generated when a new world is created |
| generator-settings | The setting used to customize world generation in this format |
| hardcore | If true, the server difficulty is hard and you can no longer play after you die |
| hide-online-players | If set to true, 3rd party programs cannot see the player list as well as in game |
| initial-disabled-packs | A list of all datapacks to disable upon world creation |
| initial-enabled-packs | A list of all datapacks to enable upon world creation |
| level-name | The world folder name that the server will load |
| level-seed | The world seed that will be created for the level name |
| level-type | The type of world that will be generated. Consists of normal, flat, large_biomes, amplified, single_biome_surface |
| max-chained-neighbor-updates | Limits the amount of consecutive neighbor updates before skipping additional ones |
| max-players | The max amount of players that can connect to the server at one time |
| max-tick-time | The maximum number of milliseconds that a tick may take before the server is stopped. -1 will disable this |
| max-world-size | The max size of the world. The default is essentially infinite |
| motd | The Message of the Day. This can be any message that is listed in the server list under the server name |
| network-compression-threshold | Compresses packets larger than this size |
| online-mode | Will check accounts joining to see if they have a Minecraft account. Only legitimate non-cracked users will be able to join in this mode |
| op-permission-level | Sets the default permission level for ops when they are opped |
| player-idle-timeout | Sets how many minutes until players are kicked for being idle |
| prevent-proxy-connections | If the connection sent from the server is different than one from Mojang's system, the player will be kicked |
| previews-chat | If enabled, you will be able to preview how your chat will look when creating your chat |
| pvp | Enables Player vs. Player on your server |
| query.port | Sets the port for the server query to get information about the server |
| rate-limit | Sets the amount of packets a user can send before getting kicked |
| rcon.password | The password for users to access remote console |
| rcon.port | Sets the port for remote console |
| resource-pack | Sets the URI to a resource pack which will be downloaded to the client when they join the server |
| resource-pack-prompt | Custom text for clients that is shown when prompted to download the pack |
| resource-pack-sha1 | Verifies the integrity of the resource pack if set to the sha-1 |
| require-resource-pack | Sets whether or not clients will need to download the resource pack before joining the server |
| server-ip | Sets the IP the server runs on at all times |
| server-port | Sets the port that the server is listening on |
| simulation-distance | Sets the maximum chunks from entities between players for them to be processed and updated by the server. Lower means less lag |
| snooper-enabled | Sets whether or not the server sends data to http://snoop.minecraft.net/ |
| spawn-animals | Sets whether or not animals can spawn in the world |
| spawn-monsters | Sets whether or not monsters can spawn in the world |
| spawn-npcs | Sets whether or not NPCs can spawn in the world |
| spawn-protection | Determines the side length of the square that protects the spawn. Set to 0 to disable spawn protection |
| sync-chunk-writes | Enables synchronous chunk writes |
| text-filtering-config | Not a finished feature as of 1.21.1, but will filter out words specified |
| use-native-transport | Optimizes packet sending and receiving on Linux |
| view-distance | Sets the maximum render distance for each client on the server. Lower means less lag but less visibility for players |
| white-list | Enables the whitelist on the server specified in whitelist.json |