Pierre342 wrote: And what if I don't have access to a garden, cause I live in a flat on the 2nd floor.

Copied from the Empire Indoor bonsai section:

Which Bonsai is best for indoors?
There are several trees that you can grow indoor, but by far the most common (and the easiest to care for) is the Ficus Bonsai. The Ficus is tolerant to low humidity and can withstand quite a lot; a good choice for beginners.
Other popular indoor Bonsai include the Crassula (Jade), the Carmona (Fukien Tea), the Schefflera Arboricola (Hawaiian Umbrella) and the Sageretia (Sweet Plum).

The Bonsai you asked about in a previous post are below from left to right: Ficus, Carmona, Chinese Elm

I have added 2 images from the net. First image is the Ulmus Parviflora leave , 2nd one is the Zelkova Serrata and my picture of my tree's leave.

it looks like the same as the Zelkova Serrata to me .