Some packages cannot be installed from Live Ubuntu USB installation [duplicate]

I have created an Live Ubuntu Desktop 20.04.3 LTS USB and booted into Ubuntu. I am confused over the fact that some packages cannot be apt-installed, such as llvm and tk-dev. I have previously installed these just fine in a regular Ubuntu 20.04.x installation and can't figure out why I cannot do it in this live boot installation.

Immediately after having logged into Ubuntu Desktop, I executed the following:

$ sudo apt update
$ sudo apt upgrade

Output of these commands can be found in this gist.

Then I am trying to install llvm:

$ sudo apt-get install llvm
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package llvm

What's strange here is that llvm is present in the official Ubuntu Focal Apt repository: see https://packages.ubuntu.com/focal/llvm and http://archive.ubuntu.com/ubuntu/dists/focal/universe/binary-amd64/ . Same for tk-dev.

When I search for llvm it clearly is not found:

$ sudo apt-cache search llvm
libllvm10 - Modular compiler and toolchain technologies, runtime library
libllvm9 - Modular compiler and toolchain technologies, runtime library
libclang-11-dev - Clang library - Development package
libclang-common-11-dev - Clang library - Common development package
libclang-cpp11 - C++ interface to the Clang library
libclang-cpp11-dev - C++ interface to the Clang library
libclang1-11 - C interface to the Clang library
libllvm-11-ocaml-dev - Modular compiler and toolchain technologies, OCaml bindings
libllvm11 - Modular compiler and toolchain technologies, runtime library
libllvm12 - Modular compiler and toolchain technologies, runtime library

Can someone help shed light on what is going on here, and why I cannot find e.g. the llvm and tk-dev packages?

OS details:

$ uname -a
Linux ubuntu 5.11.0-27-generic #29~20.04.1-Ubuntu SMP Wed Aug 11 15:58:17 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 20.04.3 LTS
Release:    20.04
Codename:   focal

$ wmctrl -m
Name: GNOME Shell
Class: N/A
PID: N/A
Window manager's "showing the desktop" mode: OFF

It seems the "universe" repository is not added in the live USB installation of Ubuntu Desktop 20.04.3 LTS.

This seems to be the remedy:

sudo add-apt-repository universe

I can then install llvm and tk-dev:

sudo apt-get install llvm tk-dev

I would argue and say this answer is related and expands further on Canonical's repositories for Ubuntu: https://askubuntu.com/a/481355/449762