In application, what is the difference between the two "otas" and "bootlader_otap", it seems that both will be related to OTAP_att_client？
I would strongly recommend you to check the Bluetooth Low Energy Demo Applications User Guide where there is a brief explanation on how to deploy the examples in the way you desire. You can find the pdf file in the documentation of the SDK.
You will need a client and a server to deploy the example you want.That is the reason why you have different OTA examples. In the example otax.. the "s" at the end stands for server and the "c" stands for client.
The board that will be updated needs an special boot loader in order to be upgraded via OTA, that is why you need the other project. You need to flash board B with the OTAP Bootloader and the OTAP Client and the board A with the OTAP Server.