If you want a nose around the standard library imports, you can currently find them in this directory after installing the canister sdk and running dfx build on a project:
~/.cache/dfinity/versions/<version_no>/stdlib
Standard Library Documentation:
https://sdk.dfinity.org/language-guide/index.html#appendix-stdlib
Download and install the DFINITY Canister SDK:
https://sdk.dfinity.org/developers-guide/quickstart.html