Hi Denis and welcome to the community forum.
I find that the easiest way to build Qobuz playlists or to add favourites is to download the Qobuz desktop player to your computer and build playlist selections or edits there. The process is so easy as you just click the heart icon for favourites or select the playlist option using the three dots to the right hand side of the desired track displayed on the album view page.
Once you have made a selection or edit using the desktop player these will automatically appear in Mosaic. Likewise any changes that you make in Mosaic will also be reflected automatically in the desktop version.
Using the desktop version is also preferable for exploring and adding Qobuz’ own playlists found on the “Discover” page such as the excellent “label stories” series.
Mosaic is great to use for Qobuz during day to day selection and play but the desktop version is superior for exploring Qobuz in full. This includes the curated aspects such as the “News Feeds” or “Panoramas” which can give an insight into unfamiliar areas of music.
So, in short, use the desktop for exploring, Mosaic for playing.