In this tutorial I explain how overflow and unchecked works in solidity

SafeMath.sol

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.12;

contract SafeMath {
    // uint8 public number = 255;
    uint8 public number = 2**8 - 1;

    function increment() public {
        unchecked {
            number++;
        }
    }

    function decrement() public {
        unchecked {
            number--;
        }
    }
}