Advanced Perpetual Exchange (APEX) is built upon the Perpetual Protocol. It leverages the perpetual futures trading functionality from the said protocol and adds additional ones, such as the ability to create limit orders and other advanced order types.
The aim of APEX was to create a decentralised and trustless system that is fully non-custodial. While other AMM based derivative DEXs rely on a trusted set up to execute limit orders, APEX's smart contracts ensure the timely execution of orders through a permissionless network of keeper-bots.
This means that at no point does APEX-PP have access to your funds, nor does it depend on trusted third parties.