Too bad you did not get an answer to this question. Have you found a solution meanwhile?
I would suggest you use Groupings to separate the students who can see some folders from those who can see others
How can you make resources available to groups/groupings? I thought this could only be done with activities?
I am new to Moodle. Is it possible to create a folder where students can save files (homeworks, essays), but only I as teacher can read the files saved in this folder?
Yes it is - either you can create assignments for them - see Assignment module - or you can even use the Database module for this - where each student places an entry but it needs moderation to be visible and you the teacher never allow it to be visible so only you and the student see the entry. See FAQ here