This page of the Resident Evil Village guide explains how to open locked Mechanical Doors. You'll find out what gadget is needed to open the door and when you can obtain it. We also explain the importance of unlocking mechanical doors.
You can come across Mechanical Doors at various places in the game world. They'll be marked once discovered on the world map. You'll be able to revisit all those doors you couldn't open before later in the campaign.
A crank is needed to open mechanical doors. You'll obtain in the Reservoir automatically, while searching for Moreau. It's not possible to miss the moment of getting the crank.
Once you've obtained the crank, you'll be able to reach all mechanical doors. Opening them can be linked to the main storyline or may enable you to discover optional locations. It is also worth mentioning that the crank has a second use - you can use it to lower drawbridges (only after reaching the bridge mechanism).