Trilium supports two different layouts, based on your preference.
The vertical layout is Trilium's original layout:
button at the bottom of the Launch Bar.
icon at the top of the Launch Bar.
The horizontal layout is a more traditional layout, since it bears similarity with other applications. In this mode:
button to the left of the first tab.
button at the end of the Launch Bar.
Go to Options and look for the Appearance option on the left. Then look for the Layout section, where there is the possibility to switch between the two available layouts.
Selecting an option will immediately apply the new layout by reloading the window.