Minecraft 1.16.5 sunflower plains seed by mountains, swamp, and birch forest
Here's a Minecraft 1.16.5 seed where you start next to a birch forest and a swamp. If you head out into the plains you'll find it transition into a massive sunflower plains biome. That eventually leads to a lot of water, roofed forest, and a mountain biome.
The seed is: -3336066758999071711