How would you do toMost quantity of products sold in the summer months (January-March) of all years (from 2000 to 2006)

You can use the month (month) and year (year) functions to establish a filter.

First, its basic use:

select   year(fecha) anio
       , month(fecha) mes
  from (      
        select cast('20180101' as date) Fecha
     ) a

Now, how to filter, in your case, would be something like:

select *
  from venta v
 where year(v.fecha) between 2000 and 2006
   and month(v.fecha) between 1 and 3 /* enero a marzo */

The rest of the adaptation in your particular case is left to the reader's exercise.

I tell you that you must do a group by, I have no where to test the query, I made it in oracle and it worked for me, when converting it for SQLServer it would be the following:

ORACLE

    SELECT to_char(fecha_documento,'yyyy-mm') FECHA, nombre_producto "NOMBRE PRODUCTO", cantidad "CANTIDAD PRODUCTO"
      FROM venta_detalle vd, producto pr, venta ve
     WHERE to_char(fecha_documento,'yyyy') BETWEEN '2000' AND '2006'
       AND to_char(fecha_documento,'mm') between '01' AND '03'
  GROUP BY to_char(fecha_documento,'yyyy-mm')
  ORDER BY 2 DESC;

SQLSERVER

SELECT      [fech_ven] AS fecha, [nomb_pro] AS tipo_producto, [cant_pro_ven] AS cantidad_producto
FROM        [dbo].[Venta_Detalle]ve_de, [dbo].[Producto]pr, [dbo].[Venta]ve
WHERE       year([fech_ven]) BETWEEN 2000 AND 2006
AND         month([fech_ven]) BETWEEN 1 AND 3
GROUP BY    [fech_ven], [nomb_pro], [cant_pro_ven]
ORDER BY    [fech_ven] ASC;

As you have very brief example, I show you what worked for me, any additional to the order

Best regards