Im using Windows 11 Pro. It has a feature called "Windows Sandbox", which is basically a virtual machine. If im getting a Virus inside of this sandbox, is it possible for that Virus to somehow "break free" from the virtual machine and also infect the host system? How safe is the Windows sandbox mode when it comes to dealing with viruses?
Possible? Yes, absolutely. Probable? No. Safe? Nearly 100% safe considering this feature is barely used or known by people. Most hackers are targeting actual VMs, e.g. VirtualBox/VMWare/Qemu, not something obscure such as a built-in Windows VM.
Side channel attacks are mostly a thing of the past unless you specifically disabled protections against them in your host OS. Besides they are not meant to escape your VM environment, but to gain knowledge of the host OS (e.g. secrets/passwords/etc).
For running unknown software I've been using SandBoxie+ which has almost a zero footprint but in theory it's a lot less secure than using a full fledged VM since it overrides system APIs.