Maybe like this?

asc = Import[
   "https://analisis.datosabiertos.jcyl.es/explore/dataset/situacion-\
de-hospitalizados-por-coronavirus-en-castilla-y-leon/download/?format=\
json&timezone=Europe/Madrid&lang=es&use_labels%20_for%20_header=true",
    "RawJSON"];

ds = Dataset[asc]

fieldNames = {"hospitalizados_planta", "hospitalizados_uci", "altas", "fallecimientos"};

ds[All, "fields"][GroupBy["fecha"], Total, fieldNames]

You could also use

data = ds[All, "fields"][GroupBy["fecha"] /* KeyMap[DateObject], Total, fieldNames]

and then plot:

data[DateListPlot, "hospitalizados_planta"]