When you start the game on Series X|S, the console will automatically detect a previous generation save game and offer to load it, allowing players to continue their game from where they left off on a previous generation console.
If you have a PlayStation 4 save game on your PlayStation 5, the game will auto-detect this and upon booting up, ask if you want to continue from this save game. To bring a save game from the PS4 to PS5, if you have an active PlayStation Plus subscription, you can use the cloud save storage to get your save game over to the PS5. The manual old-school method is that you copy the saves from a PS4 to an USB stick and import them onto your PlayStation 5. Fingers crossed!
You cannot “downgrade” a save game, meaning you cannot use your PS5 save game to continue playing on the PS4.