One business number runs your CRA accounts
The business number (BN) is the nine-digit anchor for your CRA program accounts. When you register, the CRA appends a two-letter identifier and a four-digit reference number for each account you open, giving you the 15-character account numbers you file under. The accounts in current use are GST/HST (RT), payroll deductions (RP), corporation income tax (RC), and information returns (RZ). Registering for one account does not create the others: you open each one when the need for it arises.
Source: Canada Revenue Agency, CRA program accounts (canada.ca)
The $30,000 line decides your GST/HST registration
You are a small supplier while your worldwide taxable supplies stay at $30,000 or less, measured over the last four consecutive calendar quarters and in any single calendar quarter. Cross either line and small-supplier status ends: you must register for GST/HST and charge it, with 29 days from your effective date to get the registration done. Registering voluntarily before you cross is allowed, and it lets you claim input tax credits on what the business buys. Quebec businesses register for QST separately, with Revenu Québec.
Source: Canada Revenue Agency, When to register for and start charging the GST/HST (canada.ca)
Federal incorporation protects your name across Canada
Incorporating federally with Corporations Canada gives your corporation's name protection in every province and territory, and you can locate the head office anywhere in Canada. Federal incorporation does not replace provincial registration: the corporation still registers in each province where it carries on business, though in most provinces that step is folded into the federal filing or handled with a short follow-up form. Corporations Canada also assigns your business number and corporation income tax account automatically on federal incorporation.
Source: Corporations Canada (ised-isde.canada.ca)
Hiring means a payroll account before the first pay run
The moment you pay an employee, you are deducting income tax, CPP contributions, and EI premiums and remitting them to the CRA on a set schedule. That takes a payroll deductions account, the RP account under your business number, opened before your first remittance comes due. The deduction tables change every January, and the CRA holds the employer responsible for deductions it failed to make, so the account and the routine around it come before the first hire starts, not after.
Source: Canada Revenue Agency, Payroll overview (canada.ca)