.schem is not the same as .schematic
.schem usually refers to a Sponge schematic, while .schematic is commonly a legacy MCEdit or WorldEdit Alpha file. Choose a compatible format and Minecraft version. Renaming the extension is not a conversion.
Before loading and pasting
Place the file in WorldEdit’s schematics folder, load it with //schem load filename, then use //paste. Clipboard origins affect placement. Back up your world and check permissions and server limits before pasting a large build.
Preview and convert a build
Inspect public builds on MCS or open your own local file. Convert to Litematica if you want a building overlay. A schematic containing modded blocks still needs the relevant mods in the destination game.
Common questions
Can I paste a WorldEdit schematic with a vanilla client?
Pasting requires WorldEdit or an appropriate server plugin or tool. Saving a modded structure as .schem does not turn its modded blocks into vanilla blocks.
Can I convert a legacy .schematic file?
The local tools can read supported MCEdit and WorldEdit Alpha files and convert them. Check legacy numeric block IDs, version differences and block entities in a backed-up test world.