RLCraft is one of the bigger modpacks so it will be a bit laggy, but it has probably + - 120 mods and it may be playable if you have good pc. You have to install twitch launcher and there you can download RLCraft and other modpacks.
If you trying to install RLCraft from the Curseforge platform you'll want the Twitch Launcher which you can get here: Just go to the Minecraft tab, then install it then browse modpacks, RLCraft will be one of the top ones, download it, it will get all the necessary things for it and then you can start playing once it's done.
If you want to use it on the Technic Launcher (not sure about FTB or ATLauncher), you can search for it on their website and get the link for it then put it in the search bar of the launcher or just search in the search bar and then download it, it will install it when ready and you can play when it's done.
The Twitch launcher you can create a profile with or without mods. Just make sure when making a no mods install with the Twitch Launcher that you have no modloader then it won't be modded. If a modloader version is in the dropdown then it will install Forge and it will mod the game.
For Technic I can't say for sure when creating your own modpack or non-modded as I've not gotten things to work out but you can just use their Vanilla instance and select whatever version you want.
Twitch No Mod Install
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