They sell time, in teams, with a promise attached.

A bank, a retailer or an airline needs several hundred engineers to keep its systems running, and a few hundred more to build the next thing. It would rather pay a firm a monthly rate per engineer than hire, train and manage them itself. The services giant supplies the engineers, manages them, and promises the work will be done on time and to specification. The client pays in one of three ways:

The giant's profit is the gap between what the client pays for an engineer and what the engineer costs. That gap is widest at the bottom of the organisation, where a fresh graduate costs least but can still be billed as an engineer.

The clients are the whole economy. Banks, insurers, retailers, manufacturers, telecom operators and the governments of many countries all run parts of their software on these firms' people. That is why the same firm can put one new joiner on a banking mainframe in Kolkata, and another on a mobile app in Pune.

Three words explain most of how a services giant runs: the pyramid, the bench and the account.

A client pays for engineers' time, and the giant staffs the account as a pyramid: a few leads and architects, fewer mid-level engineers and many juniors, where the margin is widest. When a project ends, engineers wait on the bench, paid but not billed, until they are placed on the next account.

The pyramid is how a team is built to make money. A client project is staffed with many junior engineers, fewer mid-level ones, and a few leads and architects, because juniors cost least compared with what the client pays for them. The shape is deliberate. It is why these firms hire freshers in tens of thousands: the bottom layer is the business model, not a training programme. It is also why a junior's first work is often narrow, such as one component, a set of tests or a support queue. The pyramid is designed for the work to be divided up.

The bench is where engineers wait between projects. An engineer who has finished one project, and not yet been placed on the next, is paid but not billed to any client. The share of engineers who are billed is called utilisation. It is one of the numbers the firms' investors watch most closely. On the bench, engineers do training modules and wait. The wait can run from weeks to months, and in a slow year, new joiners wait there for start dates that keep moving.

The account is the client engagement: all the work a firm does for one client. Once placed, an engineer belongs to an account far more than to the firm. The account decides everything the firm's name does not:

• the technology, from a 1990s banking mainframe to a cloud platform built last year
• the pace and the working hours. An account for a US client works Indian evenings.
• the manager, and how much there is to learn

This is where the name does not tell you the job. Two engineers who joined the same firm on the same day can, a year later, have nothing in common but the badge.

The same firm hires through two entrances, into different jobs.

Freshers come in through the mass-hire door, the campus and the national test, into a standard grade or a higher one, then the same training centres and an account. Experienced engineers come in through the lateral door, a job posting for a named technology, with pay set by the market, straight onto an account. Many who joined through the first door come back in through the second.

The mass-hire door is the campus and the national test. Each Indian giant runs a nationwide exam open to hundreds of thousands of final-year students, and hires tens of thousands a year through it into a standard entry grade. A smaller, higher-paid grade inside the same door, often called "digital" or "specialist", is chosen through a harder round of the same test. Both grades train in the same centres and go to the same kinds of accounts. The higher grade is more likely to be placed on newer technology, but that is a likelihood, not a rule.

The lateral door is the job posting. It hires engineers with a few years' experience and a named technology, usually for a specific client's need, straight onto an account. Their pay is set by the market for that technology, not by the campus grade. Most of these firms' job postings are for this door. Because the two doors pay differently, many engineers leave after two or three years and re-enter through the lateral door. That is when the giants lose the most people, and it is why they have started to promote engineers earlier to keep them.

Inside each giant, there are smaller units that work nothing like the pyramid. The campus process does not lead to them.

Every giant has a product-engineering or platform arm. It builds software the firm itself owns and sells, such as a banking platform, an AI platform or an automation tool. Or it does high-end engineering on a product company's own product, rather than a bank's back office.

These units are smaller and pay closer to product-company rates. They hire almost entirely experienced engineers, from outside or from the firm's own accounts. To a new joiner, the unit on the firm's website can be a different employer that happens to share its name.

In India, the ten are of a similar order of size, all within a factor of five of one another. From largest to smallest by people employed in India, they are TCS, Infosys, Cognizant, Accenture, Wipro, HCLTech, Capgemini, Tech Mahindra, IBM and LTIMindtree.

Each is known for particular kinds of work:

Largest first, by people employed in India:

Many global companies run their own engineering centres in India, called GCCs. When a bank or a retailer opens one, it often hires one of these ten firms to build it, staff it, and sometimes run it for years. So a services engineer can work in a client's building, under the client's name, on the giant's payroll. It is one more way the account, not the firm, is the job. (More on GCCs in the GCC collection.)

A global company opens its GCC in India, and a services giant often builds it, staffs it and sometimes runs it for years, so a services engineer can work on the client's systems, under the client's name, on the giant's payroll.