Players looking for Alex Caves mod 1.21 1 are usually trying to keep a newer Minecraft instance while preserving the cave content they know from the original project. The key is to understand that the original release baseline and newer ports are not the same maintenance track.
Original project baseline is 1.20.1
Original-project listings focus on Java Edition 1.20.1. That is the version to use when reading original dependency and loader information.
1.21.1 builds can be community-maintained
Newer 1.21.x versions may be available through community port projects. Those builds can target loaders such as NeoForge or Fabric, but their maintainer and technical requirements differ from the original.
Do not force a 1.20.1 JAR into 1.21.1
Minecraft internals and loader APIs change between versions. A mod compiled for 1.20.1 should not be expected to load on 1.21.1 simply because the file name looks similar.
Check the full compatibility tuple
Treat version, loader and dependency as one set: for example, Minecraft 1.21.1 + a specific loader + the exact dependency versions named by that port. If one component differs, create a new profile rather than mixing files.
Back up before migrating a world
A large world-generation mod can affect blocks, entities and generated terrain. Before moving a save from one Minecraft/mod version to another, make a complete backup and test the copy first.
Next step
Return to the main compatibility section before downloading so you can compare edition, Minecraft version, loader and file type in one place.