can we use microsoft learn during dp600 exam session?

No, the test environment that you use does not allow to have almost any other software window open. There is a check for VPN, to rigourous firewall, antivirus softwares which also have to be turned off.
It is a policy of the exam holder (not Microsoft).
On the other hand, on DP-600 exam (on special complex exams, but not on basic ones) you will have option to search on Microsoft Learn in a split (resizable) window.
Important notes:
1) there are so many questions that checking for all question is not possible, due to time limitation of the exam, so do it only for a few!
2) CTRL-F (page search) does not function in the split window, so it is important to get practice in using MS Learn search!
3) Trying to change from exam software window to another window will be recognised and either you select leaving the exam window (so the exam!) of you "stay put" 🙂 I tried it and stayed. 😄

4) For technical issues you will have someone in line (chat option) to discuss the question, but not exam questions. Of course.
5) The test/preparatory questions on DP-600 site are a good base to prepare but are not enough to pass, I would say. You either need to have practice or have to learn way more on MS Learn or other related sites. The most relevant topics are defined on DP-600 in written form and in videos of the Learn Together site.
I wish you luck!