The Base Game Mehcanics are here. Celestia used to be based on the 5e SRD, but these are being phased out in favor of the Celestia RPG Core Rules below.

Basic Game Mechanics

• Stats and Attributes
• Health
• Rolling the Dice
• Movement and Action Economy
• Status Conditions
• Skills in Celestia
• Combat Rules
• Celestia Inspiration System
• Types of Damage
• Falling
• Spells in Celestia
• Character Archetype Mechanics

The Celestia RPG Core Rules are available via a Creative Commons Attribution 4.0 International Public License. Follow the link for the legal conditions.