How to enable transcoding in asterisk??
Enable it? It’s part of its core capability. If it needs to happen, it will.
I am only giving one word of advice - avoid transcoding by any means possible. It’s a very CPU intensive process and does not scale well when the call load increases on the server.
Which core module process the transcoding in asterisk??
The codec_ modules.
which Configuration file??
No configuration is needed to enable transcoding. If it is needed and possible, it will happen. Asterisk will even disable optimisation of media paths to ensure it can happen.
The channel technology configuration files (e.g. sip.conf (for dahdi, system.conf)) can limit the choice of codecs for a particular peer, and channel variables, set in extensions.conf, can further restrict within a call. codecs.conf can add additional options for some codecs, but most are not configurable. For some codecs, you may need to provide a licence key before any transcoding into our out of them is possible. You would have needed to install those explicitly.
Generally though, this feels as though you are asking questions at the wrong level of abstraction, which is just wasting everyone’s time. Ordinary users don’t need to worry about this, and if you have a problem that you think can be solved by transcoding, you need to ask about that problem not about your proposed solution.