If in doubt, ask Siri
The biggest change to Siri in Big Sur is that it can now answer a wider range of questions by searching the web. In addition, Siri in Big Sur can tell you about your Mac’s system, so you can ask “How fast is this Mac?” It can also search for files and you can ask follow-up questions. Siri can also make suggestions to you based on how you use your Mac, such as prompt you to create a calendar event for something you discuss in Messages. Siri can also change system settings, such as turning Wi-Fi on and off, or switching on Do Not Disturb. You can search within apps, say for notes or photos, and you can launch apps using Siri, too.
Additionally, instead of speaking to your Mac to use Siri you can type questions. Go to System Preferences > Accessibility > Siri and check the box next to “Enable type to Siri.”