I used unity's player prefs to save an int after completing a level and then load the level based on that int whenever a player hits play. There are a few reasons not to use player prefs (they are easy to manipulate from what I have heard). Brackey's has a good video explaining a more secure save system.
If you want to use player prefs you can check them out here! https://docs.unity3d.com/ScriptReference/PlayerPrefs.html