Help with bitocin.conf - how to configure a testnet full node in a safe way? The general purpose here is just to have a close interaction with the bitcoin network for exploring reasons and apply some knowledge I have gained through reading here and there. 

I am not expert! So, I am afraid to mess up and enable something in the bitcoin.conf, I am not fully aware of it consequences. Especially that my personal laptop (running Ubuntu) is the only resources I have right now for a full-node setup. 

My initial goals are to:
 - run a testnet full node to experiment with the avaliable limited resources
   (4 RAM, 50 gigs free storage and metered internet connection)
 - secure my fullnode in the network (I setup the ufw firewall and created a
   new separate account no idea though if I must do more in terms of the
   account permissions. It is a standard one)

The only examples, I found are:
1) a very general file in github: https://github.com/bitcoin/bitcoin/blob/master/contrib/debian/examples/bitcoin.conf

2) This one I found in the bitcore.io documentation: https://bitcore.io/guides/wallet-service

I tried to edit it a little bit to serve my goal, I hope you can have a look into and I am more than happy to hear your recommendations. Maybe, I have to put it in another format, since it is meant to be a conf file for the bitcore implementation. I embedded some comments and questions in the file

Here it is:

{   
      "network": "testnet", 
      "port": 18333,   
      "https": true,
# must I create my keys myself? Is it the pathway important here? 
  "httpsOptions": {
     "key": "some-place-safe/key.nopass.pem",
     "cert": "some-place-safe/cert.pem"   },   
  "servicesConfig": {
     "bitcore-wallet-service": {
 # is it ok to use this port?
       "bwsPort": 3232
     },
     "bitcoind": {
       "datadir": "/home/user/.bitcore/data"
       "exec": "/home/user/bitcoin/src/bitcoind"
     }  
 },  
      "services": [
         "bitcoind",
         "bitcore-wallet-service",
        "insight-api",
        "web"   ] 
}


Probably, other stuff must be added, at the moment I am still lost and trying to navigate through the first conf file

Thanks a lot in advance!
You should use the one provided in the Bitcoin Core repository which you have linked above. That one has everything disabled by default (so only defaults are used and it will run fine out of the box and use the mainnet). All of the options that you are going to ever use are listed in that conf file with instructions on what they do and how to use them. They're all commented out.


  Maybe, I have to put it in another format, since it is meant to be a conf file for the bitcore implementation


You will need to do that because that format of conf file (JSON format) is not supported by Bitcoin Core.

The questions you asked regarding the bitcore conf are completely irrelevant to Bitcoin Core as it does not support those options.



Alternatively you can use this Bitcoin Core configuration file generator which lets you choose the options you want to enable and it will generate the file for you.