I saw a video the other day, replacing a timing chain / belt on a 2 litre diesel merc.
The mechanic was very complimentary regarding the level of engineering.
But, the sheer amount of stuff that an ICE engine comprises

No wonder a decent mechanic is like gold.
But, apart from the gunge that the mechanic pulled out of the low-mileage engine (which will be carcinogenic, btw), the thing that stuck with me was the agricultural noise it made when he ran it afterwards.
"Like a tractor, which is not far off what it is!"
And that's a modern, highly-engineered, premium marque diesel.