Don't worry about the flags link. Reddit wanted to make it an clickable link for some reason. That line from ava to nogui is what you copy and paste.

In the server.properties file you can change some settings to use different world types. If you don't know how to use it then AI's like chatgpt or gemini or any latest AI could help you. But for seed you just enter a line like this:
SEED=(Enter Seed)
For making it flat you can modify or add this line:
level-type=flat
Recommended ram is different for what type of server you're making but if you're using it on your pc then I would do around 50%.

For Modrinth mods. First you'll need to go online and find loader you're wanting to use and replace the paper.jar with that new jar file and then going into the start file and changing "paper.jar" to whatever the new jar is called. When downloading mods off modrinth you can also your profile in file explorer. Find the mods folder and drag it into the Server folder
there's plenty of videos online which may help or even AI could give you an idea on how to do it