How to customize hover-template on with what information to show

For Plotly Express, you need to use the custom_data argument when you create the figure. For example:

fig = px.scatter(
    data_frame=df, 
    x='ColX', 
    y='ColY', 
    custom_data=['Col1', 'Col2', 'Col3']
)

and then modify it using update_traces and hovertemplate, referencing it as customdata. For example:

fig.update_traces(
    hovertemplate="<br>".join([
        "ColX: %{x}",
        "ColY: %{y}",
        "Col1: %{customdata[0]}",
        "Col2: %{customdata[1]}",
        "Col3: %{customdata[2]}",
    ])
)

This took a lot of trial and error to figure out, as it isn’t well-documented, and the inconsistency between the custom_data and customdata is confusing.