It’s a flowering cherry, Prunus. Perhaps P x yedoensis but there are numerous cultivars and I know little about cherries.

Many thanks!

How an they be propagated?

They are probably propagated by grafting.

The trees in the pix look high grafted.

If somebody involved wants to get one on the cheap (instead of buying a finished specimen at a garden center) examples noticed in bloom should be visited later in spring to see if they have produced any cherries that can be planted. With it being likely that resulting seedlings will have a resemblance to the parent trees.

I have a Yoshino cherry that was heavily damaged from drought 3-4 yrs ago. Just had one limb off the trunk that survived. I ended up chain sawing the entire trunk back to the one living branch, about 7' off the ground and it looks similar in shape.

This European origin reference would be where to look to try and figure out which specific kind the one in Brussels is. The tree will have to be examined while in flower to do so:

Japanese Flowering Cherries: Kuitert, Wybe, Peterse, Arie: 9780881924688: Amazon.com: Books