okay when you save a mod, you need to save it as a .tbm. (in the input box where you save your mod type <modname>.tbm while in worldbuilder). When its done saving or whatever it will save to your scripts folder, move it from the scripts folder into the mod folder, and you can load it ingame.
Check these for more info.

Why can't I load certain mods when running a multiplayer room?

I just made a mod and want to play it with my friend, but whenever I try to load it, it loads the last loaded mod instead!

There is a few things that may solve your issue PillowTalk,

Re-download your mod where you uploaded it, it may be a bug . then wait 10 to20min before playing, also if one person in the room doesnt have the mod then unfortuntatly it wont load . So make sure your buddy also has your mod. Always make sure it has uploaded correctly too.

well got to a round press esc on keyboard then click mods double left click a mod and toda

*supabump*
Anyone have a link to all 3.9 mods?

Polkadot: The closest i could find was this
MrPopTart's mod pack, it's 205 mods, basically all of them.
I also suggest downloading the GM mod pack as well. Other than that just search up mods that you need from here.

I want to get runkido.tbm but idk how to save it

What mod is aikido with normal treshold?
I see hampa's videos on wii. that mod

how do you make a mod?

Try this thread