Schematic files can be loaded into Minecraft using a plugin called WorldEdit. There are two versions of WorldEdit, the original and FastAsyncWorldEdit (FAWE). FAWE is much better for loading large schematic files however both work. The links for either plugin are below.
WorldEdit (Original): https://dev.bukkit.org/projects/worldedit
FAWE: https://intellectualsites.github.io/download/fawe.html
VIDEO TUTORIALS
Installing WorldEdit on Singleplayer Minecraft: https://www.youtube.com/watch?v=h0vxbU8yXqE
Installing WorldEdit on a Minecraft Server: https://youtu.be/pUwRCIU0X9k