Just study them really. I took a day to learn them and it turned out easier than I thought. You could go to the Roblox DevHub or watch some of the DataStore tutorials by AlvinBloxx he makes them easy to follow too…
When the player leaves you can use Humanoid:UnequipTools() to get all the tools into Player.Backpack. After that, you can get all the children of backpack and add them to an array by adding their tool name and then use UpdateAsync() to save them.
When the player joins but has a save file, you can check that by using GetAsync() and then use a for loop to check the content of the save file. You will need to store all of the tools that you wish to get saved in a folder, in ReplicatedStorage or ServerStorage and then, for each element of the array, check if the name of the variable is found in that folder by using :FindFirstChild() and if does not return nil then you can clone the tool into the player’s backpack.
But again, you can check tutorials and I’m sure you will understand how it is done.