Do seed numbers work across platforms like Bedrock edition and Java edition?

I watched a video where the YouTuber said the seed would work on Java and Bedrock, but when I typed it in, it looked different. Did I do something wrong?

No, they (technically) won't.

All Bedrock and Java Edition seeds in the range from -2,147,483,648 to 2,147,483,647, generates the (almost) same arrangements of terrain and biomes in both Java and Bedrock editions. However, the structures and their location (such as tree's location, village's location, cave's location, etc), ore generation, and spawned entities (villager, cows, chickens, etc), as well as minor terrain, will be placed differently.

The reason why I wrote that "only" -2,147,483,648 to 2,147,483,647 (kind of) shares world, is that, Java edition has bazillions more seed - which is, 2^64, or 18,446,744,073,709,551,616 seeds.

FWIW, biome finder has the option to generate biome maps of Bedrock seeds.

Note, however, that the seed value is periodically randomized with each version change, so using the same seed value may not give the same result (no matter if it's Berdock or Java.) This tends to happen when new items, biomes, or entities are added, so in effect you can expect the seed value to change almost every update. So, if you're trying to replicate someone's world, you must set your versions same as that user's version.