Mods

I am new to the world of modding Minecraft so when I say I am completely lost on what to do, I mean I am utterly lost. To set up my modded Minecraft server for 1.12.2 I followed a youtube tutorial by the Breakdown and It works amazingly! But when I went to install the mods on the server to the mod folder and installed them all nothing happened. I loaded up my game and server and when I got on in creative mode none of the mods were present. They all work in singleplayer just fine with no crashing so I know it's not mod conflicts. I am just so confused on where to go at this point. I've tried following several threads and other videos on how to troubleshoot the problem but all the computer jargon just goes over my head and I am left more turned around than ever. All I ask is your patience and I prematurely apologize if my sheer stupidity is aggravating. Thanks in advance.

when i play singleplayer all 19 of my mods are showing up but when i enter a server i made with my friend not all my mods are showing up. on my server list theres a green check mark and when i hover over it, it says "compatible FML modded server 4 mods present" how do I make all 19 mods show up in the server? We both have all 19 installed and both of us have singleplayer worlds with all 19 working fine.    

Trying to play on a modded LAN server with my friend, when I try to join their LAN world I get the "Failed to synchronize registry data with server, closing connection" message, and they get the same message when trying to join my LAN world. We have the same forge version 35.1.12, all the same mods with the same versions of those mods as well as the same configs.   My forced crash report: https://pastebin.com/XMG5Z5Bi Friends forced crash report: https://pastebin.com/eXk8qQQT

You know in 1.15 it was so much easier with the DimensionManager class

i'm doing this mod for a modded minecraft server. I know that there is things which can support both mods and plugin like magma, but we want to use only mods without plugins.