As has been said, there are profits to be had in manufacturing, but they tend to be small and transient, as really, clicking on a button to make something takes practically no effort and minimal risk so any gap will be identified and closed quickly by the overabundance of industrial players using automated tools. However, if you can identify and capitalize on one of these imbalances before your competition, you can make significant ISK.
However, most of what people do to make their accounting show profits is actually trading (and sometimes gathering of raw materials), not manufacturing as you have identified. The industrialist will tell you their operation is profitable, but you need to sell here, or buy there with certain orders, or whatever which is actually being a trader, not a manufacturer. That’s fine and all, but don’t put too much stock in their tales of profits as most of them are terrible at tracking the time hauling, updating orders, and looking for an opportunity, and just look at the ISK/h or per production slot.
As a result, many people sell at a loss if all costs were considered, or at least seriously undervalue their time, but this is a game so as long as they are having fun they aren’t doing anything wrong. But for you, that means you have to compete with people who are willing to sell their labour too low and makes turning a profit as a pure manufacturer tough. Other than engaging in some market shenanigans (which is again trading), the best bet to make a profit as a manufacturer is to diversify as much as possibly to increase your chances of finding a transient lack of supply that you can fill.