Sideloading a world into Minecraft: Java Edition is fairly trivial, but there are a few steps to it. Worlds often contain a world folder and a Resource Pack.
Installing the World:
- Download the world
- Unzip the files
- Move the folder containing the world, that you just unzipped, into your Minecraft "saves" folder*
* If you don't know where your saves folder is located, you can check that out over at https://help.minecraft.net/hc/en-us/articles/360035131551-Where-are-Minecraft-files-stored-
Installing the Resource Pack:
- Move the folder containing the Resource Pack into the folder named "resourcepacks"
- Start Minecraft: Java Edition
- Go to "Options" -> "Resource Packs..." and move the desired Resource Pack to the "Selected" column
- Go back to the main menu and then to "Singleplayer"
- Select the world you just installed and click on "Play Selected World"