MongoDB Internal implementation of indexing?
<p>I've learned a lot of things about indexing and finding some stuff from 
<a href="https://docs.mongodb.com/manual/indexes/" rel="nofollow noreferrer">here</a>.</p>

<blockquote>
  <p>Indexes support the efficient execution of queries in MongoDB. Without indexes, MongoDB must perform a collection scan, i.e. scan every document in a collection, to select those documents that match the
  query statement. If an appropriate index exists for a query, MongoDB can use the index to limit the number of documents it must inspect.</p>
</blockquote>

<p>But i still have some questions: </p>

<ol>
<li>While Creating index using (createIndex), is the Record always stored in
RAM?  </li>
<li>Is every time need to create Index Whenever My application
is going to restart ?</li>
<li>What will Happen in the case of default id (_id). Is always Store in RAM.</li>
<li>_id Is Default Index, That means All Records is always Store in RAM for particular collections? </li>
</ol>

<p>Please help me If I am wrong. 
Thanks.</p>