Hosting plans and add-ons
At checkout you choose monthly or yearly. The choice sets the billing period for the plan, and add-ons attached to it bill on the same period and date. The price shown for each cadence is the price you pay; there is no introductory rate waiting to jump.
Every plan in one purchase shares the cadence. Buying a second plan later starts its own period on its own date.
Changing cadence
Moving an existing plan from monthly to yearly, or back, is done by support so the switch lines up with the end of the current period and nothing is charged twice. Open a ticket naming the plan.
Domains
Domain registrations and renewals are always per year, charged once at registration and then at each renewal, regardless of how hosting bills. See How domain renewals and billing work.
Cancelling mid-period
Cancellation runs to the end of the period already paid for, monthly or yearly. See Cancel a plan.