I originally tried to make it a pure d20 game and it just didn't work. The bell curve provided by 2d6 means you trend towards success, with each bonus making you noticeably better in your ability, and thus chance to succeed. The d20 was too swingy, making it so that, even with bonuses, your chance of success or failure felt random and divorced from your actual abilities.
This was great for the swingyness of combat, but felt bad for your skill rolls.
The d20 is for all saves and combat rolls, while 2d6 is for all skill rolls. This is explained on page 15, with relevant chapters giving more details.
For conversions: all B/X, 0D&D, ShadowDark, or related OSR games can have their monsters ported in at 1-to-1. Attack Scores are equal to 11+1/2 HD or LVL rounded down.
Thanks, and glad you enjoy it!