previously, it is not difficult to add library in arduino 1.8.x.
but after installing arduino 2.0, i add library difficultly in arduino 2.0 ,
first of all, i don't know route of library ,
i think the route of library is under sketchfolder in preferences menu.
especially, when i add library i made, i just copy library in libraries folder under sketchfolder.
but i can't find library at sketch> include library > contributed library section...sometimes...
and sometimes, i can find that library added at contributed library section.
By "route", do you mean the location where libraries are installed? If so, I think you got it right. Libraries are installed under the libraries subfolder of the path shown in the "Sketchbook location" field of the "Preferences" dialog (accessed via the File > Preferences menu in the IDE).
For example, If your sketchbook location preference was set to this:
C:\Users\per\Documents\Arduino
and your library was in a folder named FooLib, then a correct installation of the library would look like this:
Something important to know is that you must close all Arduino IDE windows (the most convenient way is to select File > Quit from the Arduino IDE menus) and then restart the IDE after you install the library. The library you installed manually will only appear in that menu after the restart.
Another thing that can be confusing about Arduino IDE 2.x is that, unlike Arduino IDE 1.x, it has an "asynchronous" design in that user interface components such as the menus are accessible to the user while the data that populates those components might still be in the process of being gathered. So you might find that if you open the menu immediately after starting the IDE or opening a new window, only the static items (e.g., Sketch > Manage Libraries...) are present:
If you close the menu, wait a little while, and then open the menu again, you will see the dynamic content finally loaded:
The time to load depends on the amount of data to gather (the number of installed libraries in this case) and the computing resources available (i.e., it will be almost instant for someone with a few libraries and a fast computer).
There is a report that sometimes the File > Examples menu never populates here:
I have experienced this periodically. I have not experienced the same with the Sketch > Include Library menu though, and we have not received any reports that I can find or recall.