What aspects of Prebuilt can I customize? Specifically, is it possible to add a company logo and custom branding, colors etc.
ooof there's a lot of ways to answer this. I feel like this blog post captures the amount of things that you can customize: https://www.daily.co/blog/comparing-domain-room-and-meeting-token-rest-api-configurations-for-daily-video-calls/ tl;dr: there's things you can customize through the rest api (for your whole domain or just a single room) like what language you want the UI to be in, but you can also customize the experience for a single user with meeting tokens (like let;'s say you want to give one person only the ability to record a call.
And then through daily-js you can customize tons other things like if you wanna show a leave button or a fullscreenbutton. They also have a cool guide on color and theming to match your brand: https://docs.daily.co/guides/products/prebuilt/customizing-daily-prebuilt-calls-with-color-themes#main
as far as your specific q tho, you can't add a logo directly inside but what you have to remember is that you can integrate the prebuilt frame onto your site and you can have a logo right next to it (or above or wherever it makes sense)