How to create remap insert key to command+option+I in Karabiner-Elements

Karabiner-Elements is awesome. But I don't understand modification rules.

I want to my insert key in my iMac to open Developer Tools. Can I show some examples?

I don't quite understand your posting, especially your last sentence…

But, to answer your "posting headline", you might e.g. import this complex mod. (saved as "Insert_to_DevTools.json" into your ~/.config/karabiner/assets/complex_modifications folder):

{"title": "Insert key to open Developer Tools.",
 "rules":[
         { "description": "Remaps ''Insert'' key to ''Cmd-Opt-I''",
           "manipulators":[
                          { "from": { "key_code": "insert" },
                              "to":[{ "key_code": "i",
                                     "modifiers": ["command", "option"] }
                                    ],
                            "type": "basic"
}        ]}               ]}

(This code presumes that you have a distinct "insert key". If "insert" is only invoked by pressing "shift" you'd have to add ,"modifiers":{ "mandatory": ["shift"] } after "insert".)

Once you saved this json file, import it via K-E's tab "Complex Modifications" and check [X Enable] to activate it.