I think this is basically apply
:
select v.month_year, a.sub, a.c_f, a.type, a.F_G, a.layer, v.value
from a cross apply
(values ('dec_2020', a.dec_2020),
('jan_2021', a.jan_2021),
('feb_2021', a.feb_2021)
) v(month_year, value);
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