It is very difficult to know at what level to pitch this answer so please forgive me if the simplicity insults you intelligence.
Old –Geezer is correct, your BP depends on the diameter of your blood vessels, the narrower the vessels the higher the BP, the wider the blood vessels the lower the BP, so you see that all methods of treatments concentrate on widening the blood vessels.
The state of the blood vessels depends on certain nerve and chemical(enzyme) control and it is on these that drugs do their work.
Hexamethonium, hydralazine and reserpine (derived from the medicinal plant Rauwolfia serpentina) was the first drug to be used after in the early 60’s followed by Saluric (hydrochlorthiazide) which were basically diuretics and had side effects such as potassium loss.
Then a revolution in the late 60’s.....beta –blockers...they also had undesired side effects.
Next came calcium channel blockers....cheap and few side effects...e,g Verapomil...very popular.
Then a pharmacological breakthrough.....an attack on the enzymes themselves. (ACE inhibitors )
How does the doctor know which one to start the patient on? Depends in which country you live.
Basically it depends on the degree of high BP and from then on trial and error...a drug that does the job, free of side effects and reasonably cheap.......and luckily there are plenty of choices.