Would you be so kind and add
oneplus3t: "android-armhf-64binder"
to https://cdimage.ubports.com/anbox-images/devices.json.
It will work for the oneplus3t too, if this entry is in the .json file. The devices are very similar. I have tested it with a modified anbox-tool (changed the .json url in anbox-tool to a http server of mine and used a modified devices.json).

@Kyrylo looks like that not all partitons are ext4. May be a clean start with the device can help you. It gives you best chance to do it.

• start with twrp (fastboot boot twrp)
• set twrp to adb sideload mode
• adb sideload the latest oxygenos 9.0.6 from oneplus support page
• start the phone and make initial setup of the os
• after that 'fastboot boot twrp' again
• 1st. wipe all partitions
• 2nd select wipe advanced 'change filesystem' of cache, system and data and set it to ext4
• 3rd wipe format data
• check with twrp terminal command 'mount' that all partitions are ext4. If not change filesystem again.
• connect device to usb (still in twrp) and start ubport-installer

But this is not an op3 specific question!

oneplus3t: "android-armhf-64binder"
is missing in https://cdimage.ubports.com/anbox-images/devices.json. That's why it does not work the standard way.

pascal, pl/1, abap, sql, apl, c, java, qml. maybe some more.

@alan-adale you are using the wrong recovery. Use the UT recovery for OTA updates. TWRP does not work.

@the7thstranger inside twrp you can use the terminal and make it with dd. I told you to use dd because a flash logo using fastboot is not possible and I did not know, if there is a flashable zip available. Well done.

Long: sudo always asked for the password of the logged in user who uses sudo. The user has to be in a sudo privileged group to use sudo.
Short: your own password.

pascal, pl/1, abap, sql, apl, c, java, qml. maybe some more.

@the7thstranger
Is it normal for the boot screen to not show the UBports logo?
--> yes
And is there a way to get rid of the fastboot oem (un)lock "scary" warning?
--> no, needs signed system (lineageos has signed system images).

Logo: you have to flash it yourself. Take a look here. I used it on a op3t.
You can flash the 'logo.bin' for your device from terminal (twrp) with a command like:

dd if=logo.bin of=/dev/block/by-name/LOGO

Be careful, check twice before you do it! You can brick your phone if something is wrong.

@Kyrylo I would say ADB and RNDIS are still missing. Had testet weeks ago. MTP was working (almost) for me. The most reliable ist ssh, but that does not help for tethering. Maybe it is possible to use ssh as a proxy to forward the connection from clients that can handle proxy connections (SOCKS, like firefox or thunderbird). I have not tested it, because I do not need it.

@petrmasek First of all: the hardware is getting better with every CE release!

But it could be possible to get an original UT version from pine64. Look at their homepage and ask support if you canstill get an UT-CE. In case of shipping problems because of COVID-19 it is possible,that some people have canceled their order. And my advice, pay for dhl delivery if it is available for your country or be very patient.

@Lakotaubp yes, the right hw button has features. Also the upper left silence slider has features for me. Positions: bottom=normal, middle=normal, top=silence/vibrate. I would like to see all positions with setting, mayby costomizable in an app or file.

My suggestion is: bottom=normal, middle=silence/vibrate, top=flight mode

What do you (all) think?

I have tested music (no suspend was set) over bt Speaker:
If the wifi is on the music has many clicking noises and sometimes the wifi connection was lost for short. If the phone was connected to a repeater the connection was lost permanently. Strange, had to reenter credentials.
Switching off wifi the music plays without clicking noises over bt.
Using a headphone (cable, no bt) I had no problems with the music and the wifi.
It looks like bt and wifi having problems to work at the same time. I remember I read years ago, that op3(t) has a rome chip and that there are similar problems, have no source to cite, but maybe I read it for a sailfishos port (hope one developer has time and knowledge to investigate).

@soupbowl nice to hear that. Enjoy.