The key technology is very difficult in theory? or becuse of no document or you can not access the design, such as code?
Your manager should help you, such as ask "A" document the technology, let you access the design, assign some critical work to you and let you ask question to A. You must get manager's help.
I own some key technolugies, company hired many people to back up me, I try to learn from A.
Unfortunately, I can not get help from management help.