I'm going to buy this router: https://www.alibaba.com/product-detail/product_1600644831261.html
I would like to build a firmware for it according to my needs.
Based on which firmware is it better to do this?
What exactly should I ask the manufacturer to be able to build the firmware?
build as in from source, or customize by adding you config and modules ?
firmware, or image ?
Tenbay - KuWfi T-MB5EU, if the ToH is correct.
depends, are you using "our" openwrt, or their old, molested, spin off ?
Build as in from source, by adding my config and modules 
As this: https://github.com/VladimirKalachikhin/MT7620_openwrt_firmware
but for other device.
if you're using 19.07, you're on your own, it's EOL, and there's no AX support in proper openwrt 19.07.
What is AX?
the wifi standard the product you linked to, is using.
No problem.
The 19.07 is the last which can be installed on my WE826 router. But we are now talking about a completely different device.
What target should I specify to build the last OpenWRT firmware for the specified in start topic device?
it's not, it's the latest one provided by the vendor.
WE826 is supported here - https://openwrt.org/toh/zbtlink/we-826
the Alibaba device you liked to, isn't however the WE826.
which device are we talking about, specifically ?
you've already asked the question once before, and received a reply.
It's yes, but that's not what we're discussing.
I'm sorry you couldn't help me. I'm sorry to have taken up your time.
How would I/we know?
Why bring it up in the 1st place ?
I'm sorry you're unclear, mixing two different devices ...
Should we start talking space travel too ? it's equally (ir)relevant.
See response from a developer: