On my laptop I have Linux Mint OS. Details as below:

Mint version 19, 
Code name : Tara,
PackageBase : Ubuntu Bionic
Cinnamon (64-bit)

I have installed kubectl binary using curl from this reference site: https://kubernetes.io/docs/tasks/tools/install-kubectl/ commands used for kubectl installation:

curl -LO https://storage.googleapis.com/kubernetes-release/release/$(curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt)/bin/linux/amd64/kubectl

chmod +x ./kubectl
sudo mv ./kubectl /usr/local/bin/kubectl

installed minikube on local linux machine using the link: https://github.com/kubernetes/minikube/releases command used for minikube installation:

curl -Lo minikube https://storage.googleapis.com/minikube/releases/v0.29.0/minikube-linux-amd64 && chmod +x minikube && sudo cp minikube /usr/local/bin/ && rm minikube

Downloaded this VirtualBox 5.2.18 for Linux Ubuntu 18.04 / Debian 10 AMD64 from https://www.virtualbox.org/wiki/Downloads THen I lanuched virtualbox manager. There is no virtual machine created.

Then followed quickstart guide on my local Linux Mint machine: https://github.com/kubernetes/minikube/blob/v0.29.0/README.md

On my local Linux mint machine under downloads directory, the moment I run command "minikube start" :

xxxxxxxx:~/Downloads$ minikube start
Starting local Kubernetes v1.10.0 cluster...
Starting VM...
Downloading Minikube ISO
 171.87 MB / 171.87 MB [============================================] 100.00% 0s
Getting VM IP address...
Moving files into cluster...
Downloading kubeadm v1.10.0
Downloading kubelet v1.10.0
Finished Downloading kubelet v1.10.0
Finished Downloading kubeadm v1.10.0
Setting up certs...
Connecting to cluster...
Setting up kubeconfig...
Starting cluster components...
Kubectl is now configured to use the cluster.
Loading cached images from config file.

Though I am running minikue start command on my local Linux machine, it says its starting minikube in virtualmachine inside Oracle virtualbox manager.

My first question is HOW? Is there any specific setting that is being done behind the scenes to have minikube getting started in VM installed on my operating system (in this case I am on Linux mint 19)

minikube getting started in VM

If I double click on minikube running on VM, then it opens up a screen where it asks me logging to minikube. Please see below screenshot.

minikube asking credentials

My second question is: I haven't set any credentials for minikube. What could be the credentials? Is there any way to set credentials?

UPDATE 1

As per comment by @Rico , for his 2nd point, he said "to connect to minikube VM you can run minikube ssh" isn't it same like what I showed in 2nd attached screenshot in my issue? I mean double click on minikube VM running inside oracle virtualbox manager and that pops up screen asking for login credentials.

However, I tried to use command: minikube ssh on my local machine after starting minikube.

xxxxxxxxxxxxxx:~$ minikube ssh
                         _             _            
            _         _ ( )           ( )           
  ___ ___  (_)  ___  (_)| |/')  _   _ | |_      __  
/' _ ` _ `\| |/' _ `\| || , <  ( ) ( )| '_`\  /'__`\
| ( ) ( ) || || ( ) || || |\`\ | (_) || |_) )(  ___/
(_) (_) (_)(_)(_) (_)(_)(_) (_)`\___/'(_,__/'`\____)

$ ls
$ pwd
/home/docker
$ docker images
REPOSITORY                                 TAG                 IMAGE ID            CREATED             SIZE
k8s.gcr.io/coredns                         1.2.2               367cdc8433a4        4 weeks ago         39.2MB
k8s.gcr.io/kubernetes-dashboard-amd64      v1.10.0             0dab2435c100        5 weeks ago         122MB
k8s.gcr.io/kube-apiserver-amd64            v1.10.0             af20925d51a3        6 months ago        225MB
k8s.gcr.io/kube-controller-manager-amd64   v1.10.0             ad86dbed1555        6 months ago        148MB
k8s.gcr.io/kube-scheduler-amd64            v1.10.0             704ba848e69a        6 months ago        50.4MB
k8s.gcr.io/etcd-amd64                      3.1.12              52920ad46f5b        6 months ago        193MB
k8s.gcr.io/kube-addon-manager              v8.6                9c16409588eb        7 months ago        78.4MB
k8s.gcr.io/k8s-dns-kube-dns-amd64          1.14.8              80cc5ea4b547        8 months ago        50.5MB
k8s.gcr.io/pause-amd64                     3.1                 da86e6ba6ca1        9 months ago        742kB
k8s.gcr.io/echoserver                      1.4                 a90209bb39e3        2 years ago         140MB
$

It didn't ask me any credentials. So should I say that "Minikube uses boot2docker as its base image, so the default SSH login to the VM ends up being docker:tcuser"

UPDATE 2

As per @Rico: "basically when you do minikube ssh it connects to the VM using private/public key authentication" - I agree. It didn't ask me any credentials when I did "minikube ssh" command from my host machine.

also as per point 1: "with command 'minikube start' it starts the VM that has all the kubernetes components in it" - So for this I went ahead and tried to run any kubectl command after ssh ing into minikube and its saying command not found.

$ kubectle get pods
-bash: kubectle: command not found

However it does recognize docker command

$ docker --version
Docker version 17.12.1-ce, build 7390fc6

I also tried to find VM details

$ uname -a
Linux minikube 4.15.0 #1 SMP Thu Sep 27 17:28:06 UTC 2018 x86_64 GNU/Linux