This playground runs anonymous Apex on aer compiled to WebAssembly. The runtime downloads once, then compiles and executes your code in this tab against an in-memory org with the standard objects. Nothing you type is sent to a server. Each run starts from an empty database. SOQL, DML, classes declared inline, queueables, and governor limits all work.
The runtime is the same Go program as the aer CLI, compiled to WebAssembly and run in a Web Worker. Your code never leaves the browser.
Every run gets a fresh in-memory database with the standard objects and fields. Records you insert are queryable within the run and gone after it.
Against your own repo, aer exec -p force-app '…' runs the same code with your classes, triggers, and custom objects loaded. Install aer to try it.