Ask help to understand a fundamental conception of this package

Dash Python
1

The following is the sample code:

from dash import Dash, dcc, html, Input, Output, callback

import plotly.express as px
import json

external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']

app = Dash(__name__, external_stylesheets=external_stylesheets)
fig = px.scatter(x=["a", "b", "c"], y=[1, 2, 3])
fig.update_layout(clickmode='event+select')

fig.update_traces(marker_size=20)

app.layout = html.Div([
    dcc.Graph(
        id='basic-interactions',
        figure=fig
    ),
    html.Div(id="hover-data"),
    html.Div(id="click-data"),
])


@callback(
    Output('hover-data', 'children'),
    Input('basic-interactions', 'hoverData'))
def display_hover_data(hoverData):
    return str(hoverData)

@callback(
    Output('click-data', 'children'),
    Input('basic-interactions', 'clickData'))
def display_click_data(clickData):
    return json.dumps(clickData, indent=2)


if __name__ == '__main__':
    app.run(debug=True, port="8081")

Iā€™m trying to understand where the fucntion display_hover_dataā€™s argument hoverData come from?

Is it come from the post response on url _dash-update-component?

    def _setup_routes(self):
        self._add_url(
            "_dash-component-suites/<string:package_name>/<path:fingerprinted_path>",
            self.serve_component_suites,
        )
        self._add_url("_dash-layout", self.serve_layout)
        self._add_url("_dash-dependencies", self.dependencies)
        self._add_url("_dash-update-component", self.dispatch, ["POST"])
        self._add_url("_reload-hash", self.serve_reload_hash)
        self._add_url("_favicon.ico", self._serve_default_favicon)
        self._add_url("", self.index)

        if jupyter_dash.active:
            self._add_url(
                "_alive_" + jupyter_dash.alive_token, jupyter_dash.serve_alive
            )

        # catch-all for front-end routes, used by dcc.Location
        self._add_url("<path:path>", self.index)