The only safe and official place to download SKSE is the skse.silverlock.org website or the official Nexus Mods page.
SKSE64 Build 2.2.3 (for game version 1.6.640) Nexus Mods Mirror: Skyrim Script Extender (SKSE64)
While Bethesda has since updated the game to 1.6.1130 and 1.6.1170, the 1.6.640 build remains incredibly popular because of its stability and the massive library of compatible mods. skse 16640 link
Installing SKSE isn't like installing a regular mod through a manager; it requires a manual touch to ensure the "hook" works correctly.
Make sure the skse64_loader.exe is in the same folder as your SkyrimSE.exe . It cannot be on your desktop; it must be in the game's root directory. The only safe and official place to download
Here is everything you need to find the right link, install it correctly, and troubleshoot common issues. The Official SKSE 1.6.640 Link
Essential mods like SSE Engine Fixes , Address Library , and Dynamic Animation Replacer work perfectly on this build. Make sure the skse64_loader
Many modders intentionally "backport" or freeze their game at 1.6.640. This is because the transition from 1.6.640 to 1.6.1130 involved significant changes to the way the game handles plugins (the "Marketplace" update).