It depends on the project. I’ll give a few examples by trade/discipline.In a hospital project the biggest cost is probably the mechanical system (including dry side, wet side, and med-gas) but this depends on the project and how you break out the various trades. This excludes the OFCI (owner furnished contractor installed) medical equipment which can get very expensive but from the construction side is only the cost of installation.
For a data center the biggest cost is in the electrical and telecom part. These are basically just big sheds with thousands of computers, massive networking, and megawatts of power distribution.
In bridge construction the structure is obviously by far the biggest expense though on difficult sites the site work, make ready, and temporaries can really add up. If you want to split out structure between concrete and steel dams are obviously heavy on the former and long span suspension or truss bridges on the latter.
In waste treatment plants plumbing is paramount for obvious reasons.
In manufacturing plants the biggest cost is typically the installation of the equipment and machinery. Assuming a modern automated assembly line just installing all the stuff is a major expense even leaving out the cost of purchasing the equipment. This is such a specialized trade that I’m not even sure what the name is for it.
For semiconductor fabrication it is probably the clean room requirements. This is arguably the biggest cost since it pushes the cost of fabricating and installing all of the otherwise very expensive equipment much higher than in conventional factories. This is not a specific trade and discipline but a specialization for all the trades and disciplines involved.
Prison and detention facilities’ biggest cost is security. No big surprise. The security requirements drive up the cost of everything in the place.
Major storm water management and infrastructure projects are very heavy on earthworks costs. Individual bridges are heavy on structure but, cutting, moving, and replacing millions of yards of rock and soil and prepping miles of roadbed, retention ponds, and so on will typically eclipse the other odds and ends. Except of course in densely populated areas where every cut and fill gets meticulously retained, encased, drained, irrigated, and landscaped (ideally). This is the playground of civil and operating engineers.
I can’t think of a project type where fire protection is the main cost and there are plenty of even more specialized trades and disciplines which are unlikely to be the biggest cost in any project.
Interiors and finishes may be the biggest cost in some retail, entertainment, and hospitality projects but this would be on a case by case basis.