This is the easiest and most stable way to modify your game. Resource packs change the look of blocks, items, and the GUI.
Most "client-side" mods work on servers, but "server-side" mods (like New Blocks) require the server itself to be specifically modified.
If you want, I can:
These tutorials provide visual steps for setting up mod loaders and using custom modded clients: How to install eaglerforge(Mod loader for eaglercraft) 76K views · 1 year ago YouTube · red stick (radmanplays) How to put mods in eaglecraft 1.12.2 9K views · 8 months ago YouTube · Aventuras Minecraft ⚔
How they work: They hook into the browser's developer console or use a userscript manager like Tampermonkey.