I have now installed Ubuntu three times. However, I dont know how to start Ubuntu and cannot find any trace of Ubuntu on my laptop.
Does anyone have any idea what I'm doing wrong? Anything else I could try?
Here is the list of (I guess) relevant details:
X1 Carbon 10th Gen (Type 21CB), with Windows 11 (current, reinstalled last week with all updates)
ubuntu-24.04.1-desktop-amd64 (latest version)
ISO image written to new USB stick with RUFUS (latest version).

Ubuntu installation (as a second OS, alongside existing Windows) successfully completed (only WiFi connection cannot be established).

When I restart, I can't select Ubuntu instead of Windows anywhere. It only ever starts Windows.
Only Windows is available in System Configuration > Boot:

Also in the boot menu only Windows is available:

Secure Boot is off:

Detail: My Windows obviously doesn't have the quick start function. Startup always takes about 30 seconds.
EDIT
Meanwhile I bought a second SSD to install Ubuntu as the only OS, hoping to make things simpler.
Again, the installation from USB-Stick seems successful (this time WLAN connects immediately).
However on restart the laptop goes in an endless loop of restarting, Ubuntu doesn't start.
Restarting with USB-Stick plugged starts the installation process again. However if I cancel the installation menu window I after the media validation black screen I am suddenly in the Ubuntu desktop. Nothing here persists to the next restart of this type.

sudo fdisk -l

sudo lsblk -o
lsblk -ape7
Bootinfo summary report https://pastebin.com/wnBXmcqU
When I continue with the installation I am forced to manual partitioning and I see these partitions (I did not partition deliberately anything before). Here the continue button remains inactive, I cannot continue, no matter what I choose, including after overwriting new empty partition

and later

I read on ubuntu.com that Lenovo X1 Carbon Gen10 is not officially on the list of verified devices for tha lates two releases.
Boot-repair summary report: https://paste.ubuntu.com/p/MG6kwFrrn7/
Installing linux should be easy and straight forward, isn't it? How is all that possible? I'm in disbelief.