Here, I've put together a little guide about how to install Debian Bookworm SailfishOs 4.6. This guide covers both arm32 and aarch64 architectures. (Two examples for the aarch64 architecture: Motorola G7 Power (ocean) and Sony Xperia XA2.)
In this instruction the container name is: Debian

As all steps must be done in command line, I suggest to do it via ssh because typing all in Fingerterm on the device is very difficult and expose risk of typos.

1. Add chum repo and install Chum-gui app as root:
If you already have Chum-gui installed, you can skip this step.

2. Install LXC, qxcompositor, containers app and some other as root:

3.1. Create container and install debian as root:
When it pops up a few questions, answer to those with:
Distribution: debian
Release: bookworm
Architecture: arm64 (or armhf)

3.2. Add the following two lines into /var/lib/lxc/Debian/config file as root:
(Without this, there won't be /mnt/guest folder within the container!)

3.3. Start container as root:

3.4. Attach container as root:

3.5 Now you should be within the Debian where execute two .sh scripts:
(this will take for a while and will ask to chose desktop environment (I used xfce) and root password. (When the script is finished it will quit and stop the container.)

3.6. Execute the next script which originally was meant to download & compile Xwayland binary. Unfortunately, that bit doesn't work anymore, so we will download the Xwayland binary in a different way. However, this script still should be executed.
Start and attach & attach the container, then execute the second .sh script as root:
(When the script is finished it will quit and stop the container.)

3.7. Download Xwayland.xxxxx.libc-2.29 .bin binary: Xwayland for armhf(32) or Xwayland for aarch64
Rename the downloaded file to Xwayland, then copy the Xwayland into the container as root:
(if bin folder doesn't exist there you'll need to create it)
then set it to be executable:
3.8. Now you are good to go.

Start Debian

1. I wrote a .sh script to easily start the container from command line. It can be used in a .desktop file, so can be there as a native app. Download the debian-mouse.sh file Copy the debian-mouse.sh into your home and make it executable:

2. For starting the container you just need to execute the script.
It has two parameters:
$1 - your container name (e.g Debian)
$2 - layout: p (portrait) or l (landscape) or a (auto-rotate)
Example:

3. The container should open as an app.

Update:
If you'd like to have an icon in your app grid and start your container as an app, please download lxc-debian.zip file, extract it and as root copy its content to the relevant folders on your Sailfish system.
This is how it looks on my XA2: