Keep in mind that if you're using a Homebrew version of something Apple ships with macOS, for now, you'll get x86-64 binaries while Apple's version is a universal app and will run natively on the M1.

Zsh from Homebrew: /usr/local/bin/zsh: Mach-O 64-bit executable x86_64

Apple's Zsh:

    /bin/zsh: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit
    executable x86_64] [arm64e:Mach-O 64-bit executable arm64e]
    /bin/zsh (for architecture x86_64): Mach-O 64-bit executable x86_64
    /bin/zsh (for architecture arm64e): Mach-O 64-bit executable arm64e

The Homebrew team is making progress; I've been seeing Big Sur "bottles" for a couple of days now.