that point is nearly impossible to define
As with any law, there will undoubtedly be cases in which it is difficult to discern whether or not a law has been broken, but courts decide on innocence or guilt in such cases every day. A jury would be asked to decide whether a a reasonable third party is likely to conclude on the balance of probabilities that the image depicts a person who is under 18.
Whether or not the depicted person is real or imagined is not relevant in many / most jurisdictions.
The “when transporting a large number of people” is quite a caveat. Sure ok high saturation of public transport / walkable cities is probably achievable with high population density, but in rural / regional areas it’s just not possible.