Hello, schematics must be placed in your .minecraft>schematics folder. This folder can be accessed on Windows by activating the run command by pressing the windows key and R at the same time. Then type: %appdata% exactly and press enter. Next your file explorer will open. Then you will want to click on .minecraft. then click on schematics. lastly drop the .schematic file into the schematics folder. Now to actually paste it in your world, the easiest way is to use a client side mod like litematica or world edit to paste it into your world. Keep in mind that both of those mods are for fabric and thus will require you to run the game in fabric. There are plenty of tutorials on how to add the schematics to your world for both mods on YouTube. If you have any follow up questions, feel free to ask.