Opening Google/YouTube URLs in Chrome by default when Safari is default browser

As of macOS Sierra there is no way to do this without a a little proxy program which you set as the default browser. This proxy program then dispatches the URL according to some settings and URL rules to the desired browser application.

Building on tron_jones's comment, I know of three available proxy programs you can use:

• LinCastor - Free: Doesn't seem to be currently maintained
• Choosy - Commercial: Very slick. Easy to setup. Great UI
• brosel - Open Source: Very lightweight. Setup only for people who are comfortable with the command line. (My own little contribution to this field...)

You would need one rule for the general case selecting Safari by default and a second rule for Youtube URLs which chooses Chrome. Specific setup depends on the tool you are using.