JSON is a data format for serialisation, not a way of saving the game. It’s not really comparable to player prefs.
Saving is a three step process.
Gather the data to be saved
Serialise the data
Save the serialised data
Step one is very game dependent, so general advice won’t help.
Step 2 is where JSON comes in. Other common methods include XML, binaryformatters and csv. The idea is to convert C# variables into something savable.
Step 3 is where player prefs come in. Other methods include local files, servers, or sometimes just placing the data on screen for the user to copy paste.
Point is JSON doesn’t actually save your data. It just gets your data to a point that it can be saved. Think of it as the words to a story, you also need something to write the words on. So you still have to dump the JSON to player prefs, a web server, a file, or something.