[Bro] How can i find some fields?

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first of all, please understand my poor English. Because I'm not live in English-speaking country.
I'm interested in Bro and Bro scripts nowadays. It's very flexible.
But, I don't know how to create a module's fields. For example, DNS module in bro has some fields of "Info".

===================== base/protocols/dns/main.bro
module DNS;

export {
        ## The DNS logging stream identifier.
        redef enum Log::ID += { LOG };

        ## The record type which contains the column fields of the DNS log.
        type Info: record {
                ## The earliest time at which a DNS protocol message over the
                ## associated connection is observed.
                ts:            time               &log;
                ## A unique identifier of the connection over which DNS messages
                ## are being transferred.
                uid:           string             &log;
                ## The connection's 4-tuple of endpoint addresses/ports.
                id:            conn_id            &log;
                ## The transport layer protocol of the connection.
                proto:         transport_proto    &log;

………..last part omitted…………
=====================



I have tried 'SMB' module. But I didn't know where i can get fields.

here is my question.
1. Can I get protocol fields? (such as DNS, HTTP, SSL module's Info fields)
2. Where I should to find the fields? (source code?)
3. If it is not possible, how can some people write a bro script? 
like these 
https://gist.github.com/maxfeldman14/brospects/tree/4f9aee880234bd27d9fcab82a7410fa455ddaf42
https://github.com/sheharbano/scan/blob/master/scan.bro

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