Sideloading or uploading a world into Minecraft: Java Edition is easy, but there are a few steps to it. Worlds often contain a world folder and a Resource Pack.
Installing the World
- Download the world save that you would like to play on.
- Unzip the files.
- Move the folder containing the world you just unzipped into your Minecraft saves folder.
If you don't know where your saves folder is located, you can check that out at 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 then select Play Selected World.