array_sum(
array_foreach(
generate_series(0, layer_property('MY_LAYER', 'feature_count'), 1),
if(attribute(get_feature_by_id('MY_LAYER', @element), 'MY_FIELD_TO_FILTER') = 'MY_FILTER_VALUE',
attribute(get_feature_by_id('MY_LAYER', @element), 'MY_FIELD_TO_AGGREGATE'),
0
)
)
)
is the same as
aggregate(
layer:='MY_LAYER',
aggregate:='sum',
expression:="MY_FIELD_TO_AGGREGATE",
filter:="MY_FIELD_TO_FILTER"='MY_FILTER_VALUE'
)