Use variable as column name

create or replace function add_stats_to_player() returns trigger as $add_stats_to_player$
    declare
        equipment_id int := new.player_equipment_armor['armor_id'];
        equipment_stats varchar[] := array(select jsonb_object_keys(armor_stats['stats']) from equipments_armor where armor_id = equipment_id);
        equipment_stat varchar;

    begin
        raise notice '%', equipment_id;
        foreach equipment_stat in array equipment_stats loop
                if old.player_equipment_armor['stats'][equipment_stat] is not null then
                     update players set equipment_stat = equipment_stat - old.player_equipment_armor['stats'][equipment_stat] + new.player_equipment_armor['stats'][equipment_stat] where player_id = new.player_id;
                end if;
        end loop;
        return new;
    END;
$add_stats_to_player$ language plpgsql;

create trigger add_stats_to_player after insert or update of player_equipment_armor on players
    for each row WHEN (pg_trigger_depth() < 1) execute function add_stats_to_player();

create or replace function add_stats_to_player() returns trigger as $add_stats_to_player$
    declare
        equipment_id int := new.player_equipment_armor['armor_id'];
        equipment_stats varchar[] := array(select jsonb_object_keys(armor_stats['stats']) from equipments_armor where armor_id = equipment_id);
        equipment_stat varchar;
    begin
        if (TG_OP = 'UPDATE') then
            foreach equipment_stat in array equipment_stats loop
                    if old.player_equipment_armor['stats'][equipment_stat] is not null then
                       execute 'update players set ' || equipment_stat || ' = ' || equipment_stat || ' - ' || old.player_equipment_armor['stats'][equipment_stat] || '+' || new.player_equipment_armor['stats'][equipment_stat] || ' where player_id = ' || new.player_id;
                    else
                       execute 'update players set ' || equipment_stat || ' = ' || new.player_equipment_armor['stats'][equipment_stat] || ' where player_id = ' || new.player_id;
                    end if;
            end loop;
        elseif  (TG_OP = 'INSERT') then
            foreach equipment_stat in array equipment_stats loop
                execute 'update players set ' || equipment_stat || ' = ' || new.player_equipment_armor['stats'][equipment_stat] || ' where player_id = ' || new.player_id;
            end loop;
        end if;
        return new;
    end;
$add_stats_to_player$ language plpgsql;

create trigger add_stats_to_player after insert or update of player_equipment_armor on players
    for each row WHEN (pg_trigger_depth() < 1) execute procedure add_stats_to_player();