Old shipbuilders of old bent wood with water and steam and then large peg jigs and winches.
This still works to an extent, but depending on the size, I have bent two 2x6 end to end to make a perfect half circle for a deck.
What I did was to mark every inch on the side of the board, the 5 1/2 inch side, then set my circ saw for about a 3/8 inch deep cut. Then make those cuts all along the side. Then just for safety sake, set three heavy pegs into the earth and bowed the board in the position needed. Then kept it wet all day while doing other jobs.
Then fit the boards end to end to make my arc. I had to be extra careful in nailing not to crack the board so I pre drilled all holes. Do not nail into a kerf.
with this method, I would think you could not bend anything bigger than a 2 by. You can bend a 2x4, a 2x6, or 2x8 or any 2by, but not anything that is a 4 by or larger.