So where / how does this data gets stored or sent to?

Question

<label> Username </label>
<input type="text" name="username" placeholder="Username"/> <br />
<label> Password </label>
<input type="password" name="password" placeholder="Password" /> <br />
<input type="submit" name="sumbit"/>

what type of coding course will i need to make/create a database?

Answer 1

When the user presses the submit button the data will be send to through http/https request to the webserver which will handle the request. After the request is handled the data will be stored in a database and the user may also receive a confirmation that the data has been successfully stored.
To fully undestand what's happening you need to learn a backed language like php. You could also learn NodeJS which is a runtime envirorment for JavaScript.
To work with databases you need SQL. Sololearn offers a basic SQL course and I would highly recommend if you want to learn how a database works.