How does an engine get metal shavings down inside of it?

Hello,

If the head was machined and not cleaned properly it can be the cause of metal shavings. Also keep in mind that many times new engine components can create metal shavings.
Keep in mind that microscopic metallic shavings are not a cause for concern unless substantial.
They are usually picked up by the oil filter while traveling through the lubrication system. If you easily visible and excessive metal shavings you need to to an engine flush and oil change to get them out before it cause damage.

Here is a video that you might find helpful:

https://youtu.be/MnqlO3OhNGA

Please let me know of any questions.

Thank you.