Secure Boot is a feature of your computer's UEFI that only allows approved operating systems to boot up.
Defender will continue to provide security even for unsupported Windows 10 PCs. But Microsoft still wants you to move to Windows 11.
You can choose to upgrade to Windows 11 for free if your computer is compatible. You can buy a new PC that already has Windows 11 pre-installed (or opt for an alternative, like a Mac or a Chromebook).