@gaigous - most technological processes used by isekai protagonists are the kind that are hard to get wrong. For example, you don't really need to know the exact proportions for making soap, you'll end up with some soap-like stuff anyway.
The more advanced ones are either made by people who have actual skills, by people who have extraordinary access to knowledge, or with super-materials that make the underlying technology easier. For example, Magi Craft Meister was a quite capable worker in his earthly life, and builds airplanes out of mithril, which is stronger and sturdier than materials available to early aviators.
@Acolytus - people know a lot of random technological stuff. I usually assume the MC just happens to know the ones he made, and not know the ones he did not make. It's quite fun - learning that stuff you normally can only find in a shop can be made without high-tech factories.
From my research, things that cannot be made by isekai protagonists start around the level of smokeless gunpowder.