The "brain" of the ASA is the Cisco ASA Software, a proprietary operating system that runs on specific hardware architectures. Historically, ASA software ran on custom Cisco hardware (like the ASA 5500 series). However, as the industry shifted toward Network Function Virtualization (NFV), Cisco released virtualized versions of the firewall. This evolution allows the ASA image to operate as a Virtual Machine (VM), behaving exactly as it would on physical hardware but abstracted from the underlying physical components.
The fastest route is downloading a virtual appliance that is ready to run. Many educational repositories offer a .vmdk hard disk file paired with a .vmx configuration file. cisco asa firewall image for vmware workstation
Cisco Secure Firewall ASA Virtual Getting Started Guide, 9.16 The "brain" of the ASA is the Cisco