Hi Fouad,
You’re correct that you cannot find the flex container left in the building blocks.
Maybe you can report this to the Mendix academy that this isn’t possible?
In Mendix Studio Pro v9.12.2 you can use these building blocks.
Hey Fouad,
Try downloading Atlas web content module,
Mendix Marketplace - Atlas Web Content
I am not sure it can solve your issue, but it should.
Hope it helps!