brand identity is how you want the market to perceive your product or brand. includes elements like colours, design, logo, name, symbol.
The identity system usually starts after the logo is complete. The purpose of the identity system is to form a systematic visual language around the logo.
The style guidelines contain and prescribe the logo usage rules, typeface system, color palette, layout guidelines, and more.
The process of getting concepts down on paper by sketching out dozens if not hundreds of iterations then start iterating on them digitally.