In Windows 10 and Windows 11, you already have Windows Security which is free and that take care of security of your PC and protect you against malwares. In this case, if you subscribe to Microsoft Defender, it won't install another Anti-Malware product and use Windows Security which exists inside the Windows but add additional features like ability to monitor security status on phone and so on. Same for iOS, it will not provide Anti-Malware solution for iOS. It mentioned "Once downloaded, the Microsoft Defender app extends the device protection already built-in to Windows Security beyond your PC to your mac, iOS, and Android devices." Also have a look at the following in the website: "What is the difference between Microsoft Defender for individuals and Windows Security?
Microsoft Defender for individuals is a cross-device app that helps individuals and families stay safer online. Defender provides a simplified security dashboard, antivirus protection, antiphishing protection, identity theft monitoring, credit monitoring, and VPN. Defender for individuals brings device protection to iPhones and iPads, Android phones and tablets, PCs, and Macs. Defender for individuals is available in the Apple, Google, and Microsoft app stores, as a direct download for Mac, or accessible via the Microsoft Defender web portal. Defender for individuals requires a Microsoft 365 Personal or Family subscription to use.
Windows Security, (formerly known as Windows Defender Security Center), is security built into Windows to help protect your device and data. Windows Security is pre-installed and automatically enabled. Windows Security includes Microsoft Defender Antivirus software that helps protect your Windows device and data against viruses, ransomware, trojans, and other malware unless a non-Microsoft Antivirus solution is active. Windows Security virus and threat protection status is displayed within Microsoft Defender to provide a unified view of your security status, across all your devices. " Because you already have Windows Security which contain Anti-Malware protection it won't install additional Anti-Malware product in Windows and reuse and extend it. It is not Enterprise version and is for personal use to manage few PCs.