Adding a few more pics.

Quartzite is a naturally occurring product and you are most likely not changing nature. That's what it looks like to me anyway.

Visible lines in quartzite countertops are often natural, non-structural, and textural features called fissures, which are part of the stone's, such as Taj Mahal quartzite, natural makeup. Dark, wet-looking lines, especially on edges, are usually temporary moisture absorbed during cutting, not permanent stains. Other visible lines include structural seams (where slabs are joined) or, rarely,, cracks.

Thanks. These (to me) seem to have nothing to do with the stone’s structure but rather the fabrication. Almost like if it was etched (although quartzite isn’t supposed to etch)

Well that doesnt look like natural stone to me. looks to be quartz snd possible markings from transport.

Thanks. I don’t think it is quartz but regardless, if the markings are from transport, is there a remedy that doesn’t make a big mess?

Looks like quartz to me - what is the name of the exact product? Did your fabricator seal it after installation?

It is Milano Quartzite.

That is a dolomitic marble, not a quartzite. It's etched from some bath product.

Not all slabs labeled as quartzite are true quartzites. Ideally you would have done some test on the slabs before you bought them. Search for old Geology threads by karin_mt for easy tests.

Most of the white/gray "quarzites" can be hard like a true quartzite, but porous like marble, so they etch and stain, and will need to be sealed.

Buyer Beware.

If the slabs are marble/dolomite and they were stored outside, they might also have gotten stained/etched while being exposed to the elements.

Were these stains there before it was fabricated?

When you saw the slabs were they cleaned up or covered in the usual slab yard dirt/dust? If not clean, you might not have noticed the weathering/etching that was already there, specially when seen in a vertical position in poor warehouse lighting.

I always ask to have my slabs taken outside in natural light and to be wiped down ... and even then you can miss some stuff!

Im still not convinced it's quartzite - this product is called 'Milano Quartzite' and it is indeed a quartz not a quartzitehttps://www.stonecontact.com/products-789421/milano-quartize-white-slabs

Get a stone restoration professional to give it a go, please.

Where is the original, full slab? any pics?

where does the seller say this "quartzite" came from? many places mislabel marble/dolomite as a quartzite.

quartzite will never etch, marble/dolomite will.

quartz (countertops) is man-made, quartzite is mined.

many places also rename their slabs whatever they like. Milano doesn't necessarily mean it's the "market name for this type of stone". it just means the seller picked it.

were those marks there before fabrication? did you show them how you wanted it cut?

need more info