By the time they retire, most people will have worked for a number of employers, and many will have built up a number of different pensions. If you have a number of different 'money purchase' (DC) pots, you may want to combine these before you buy an annuity. You will probably need a financial adviser to help you with this.
If you leave a final salary scheme and don't retire until later, you will receive what is known as a deferred pension (also called a 'preserved pension'). This will be worked out from your salary when you leave the scheme and your years of service in the normal way, and then uprated in some way to take account of inflation for the years between leaving the scheme and when you claim your pension.
The law sets a complicated set of minimum standards for these increases, and most schemes will do no more than comply with the legal requirements. If you have lost track of a previous employer's scheme, the Pension Tracing Service on the GOV.UK website can help.