Are Minecraft seeds the same on Java and Bedrock?

Since 1.18, mostly yes. Before that, mostly no. Since Caves & Cliffs, both editions build the Overworld from the same noise, so the same seed gives you the same mountains, rivers, oceans and biomes on phone, console and PC. What's on top of that land is a different story: Bedrock places villages, temples, strongholds and everything else with its own code, and the same goes for the Nether and the End.

That's why a "village at spawn" seed from a Java list often has no village at spawn on Bedrock, and the other way round. Seeder does both: pick your edition with the switch, and the map, the finder and the tools show you what your edition generates.

What stays the same (1.18 and later)

The Overworld biomes and the shape of the land. Bedrock uses exactly the same noise as Java, bit for bit, so the biome map is identical. The spawn point is close too, usually within about 20 blocks, since it's picked from those same biomes.

What changes

Structures. Villages, pillager outposts, temples, monuments, mansions, shipwrecks and the rest are placed with Bedrock's own random numbers, which only read the lower half of the seed, so they end up somewhere else. Trial chambers, trail ruins and abandoned camps are the exception: they're placed like on Java, but Bedrock still skips or keeps different ones depending on the terrain.

Strongholds. Java puts them on rings around the center of the world. Bedrock puts three under villages near spawn and the rest on a grid, so "stronghold under spawn" seeds never carry over.

The Nether and the End. Bedrock seeds both from the lower half of the seed, so Nether biomes, fortresses, End islands and End cities are all different.

Slime chunks. On Bedrock they don't depend on the seed at all: every world has the same ones. On Java they do.

Before 1.18

Nothing matched. Bedrock had its own biome layout, so a 1.17 seed gave you two completely different worlds. That's why Seeder's Bedrock mode starts at 1.18.

How to check a seed on your edition

Open it on the seed map and pick Java or Bedrock with the switch: the structures you see are the ones your edition generates. The structure seed lists also have a Bedrock version, searched on Bedrock, and the finder follows each edition's own rules. Everything about Bedrock is on Seeder for Bedrock.