How can I select only some of the parts of sound, so first and 3rd part :
Currently, Audacity supports selecting only one time region at a time. The selection may span multiple tracks.
More info about Selections: https://manual.audacityteam.org/man/audacity_selection.html
I did read the selection info but I couldn’t find how can I select specific chunks of sound.
With selection tool F1 I can be very inprecise
Selecting with the mouse (with the “F1” selection tool) is generally the quickest way to make an audio selection. For increased precision, zoom in on the end of the selection, then use “Shift + left click and drag” to adjust the end of the selection.
I usually use “Ctrl + Mouse wheel” for zooming in/out. Other methods are available: Zooming Overview - Audacity Manual
If you need the selection to be at an exact time, then the “Selection Toolbar” allows you to enter exact times: Selection Toolbar - Audacity Manual