I'm reasonably sure their laminate kitchen countertops (Saljan, Ekbacken) are solid Laminated MDF and the Ekbacken comes in matte anthracite. https://www.ikea.com/us/en/p/ekbacken-countertop-matte-anthracite-laminate-30409021/#content
According to what someone people have commented I think the newer Alex Drawers come with pre-drilled holes to mount them to the countertops. I bought mine a while back so I marked and pre-drilled my own holes and used 1-1/2 countersunk screws to secure the tops of the Alex drawers to the Saljan top I used. I also used recency table stiffeners to prevent sag. They come in two lengths, and I ended up using the 60" ones and cut them to fit (about 45.5")
46" - https://www.amazon.com//dp/B01DOZDUEK/
60" - https://www.amazon.com/dp/B01DOZDUKO/
Here is them along with the tops of the alex drawers mounted to my Saljan Top.
https://i.imgur.com/Zy79nrT.jpg
I don't have much in the way on insight on how to connect the two tops to make an L shaped desk unfortunately, or how much support you might need there.
for supporting the underside?
take a look at this: https://www.amazon.com/gp/product/B01DOZDUEK/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
i've got a single one under my 74" saljan and its very solid. two would be ideal
You could always go with something like these. https://www.amazon.com/dp/B01MD2ILSH/
Then if you wanted added stiffness to the top to help with potential sag, you could add these to the underside. https://www.amazon.com/dp/B01DOZDUEK/
Regency Tables Stiffeners. Used them instead of a center support leg.
https://www.amazon.com/dp/B01DOZDUEK/
https://www.amazon.com/dp/B01DOZDUKO/
Here is them mounted on the underside of the Saljan top I used. I only had the 60" available at the time I built my desk and had to cut them down to 45.5" to fit in between the two Alex Drawers.
https://i.imgur.com/Zy79nrT.jpg
This is my setup with a 34" Ultrawide & 27" Widescreen in Portrait.
Yeah, it is attach to the Karlby in the back 1/3. Used this instead of a leg to support monitor.
https://www.amazon.com/dp/B01DOZDUEK?psc=1&ref=ppx\_yo2ov\_dt\_b\_product\_details
