How can I access the map in Sekiro pc version?

There is no "level map" in the conventional sense in Sekiro. It is possible that the player you watched edited a map into the video or opened it in another tab.

However, there is a map called the "Antique Map" that can be accessed by pressing the "Y" or triangle button on the controller while you are in the travel screen. On PC, the default key to open this map is F. Look in the bottom right hand corner of the travel screen image to see the tooltip. I have included a spoilered picture of both the travel screen and map below.

Note: The Antique Map does not start with all of the areas named. The area names are filled in as they are discovered.