Unknown location of WordPress themes in React - reactjs

Question

I downloaded and was running this React project. It compiled successfully and it works for me.
But see the following:
<div className="jumbotron">
<h4>Login</h4>
{ error && <div className="alert alert-danger" dangerouslySetInnerHTML={ this.createMarkup( error ) }/> }
<form onSubmit={ this.onFormSubmit }>
<label className="form-group">
Username:
<input
type="text"
className="form-control"
name="username"
value={ username }
onChange={ this.handleOnChange }
/>
</label>
<br/>
<label className="form-group">
Password:
<input
type="password"
className="form-control"
name="password"
value={ password }
onChange={ this.handleOnChange }
/>
</label>
<br/>
<button className="btn btn-primary mt-3" type="submit">Login</button>
<p>{ loading && <img src={Loader} className="loader" alt="Loader"/> }</p>
</form>
</div>
Where are btn-primary, alert-danger, etc. defined?
Thanks for any help.

Answer 1

The classes btn-primary and alert-danger are part of bootstrap, a framework for developing websites. You can find the docs for the buttons here; for the alerts here.
The bootstrap css file is loaded in the index.pug file.
link(rel="stylesheet", href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css")
Note that this has absolutely nothing to do with wordpress. Bootstrap is a completely independent framework and can, as such, be incorporated in any website whatsoever, wordpress or not.