What do you call someone who's fascinated by machines?

See people who are fascinated by machines like computers, cars, bikes, robots. They adore engines, hardware the visual looks for them the smell of it, everything about it.

Such people are hardware maniac.

From Merriam-Webster...

technophile - someone who likes and enjoys technology and modern machines (such as computers)

More informally, there's gadgeteer, gadget man, gadget freak, etc. (gadget - a small, unique-use mechanical or electronic device).

Personally, I often also use propeller-head (often disparaging: an enthusiast of technology and especially of computers), but that may not be so common among Anglophones in general.

Gearhead (as per Merriam Webster). But in common usage it more accurately describes someone who wants to possess such equipment.

: a person who is very interested in mechanical or technical things (such as cars or computers)

Another, but even more idiomatic, is Propellerhead.