Two bikes with the same number of gears can still feel very different on a hill, because what actually matters for climbing ease is the gear ratio, not the total gear count printed on a spec sheet.
A gear ratio compares the front chainring's tooth count to the rear sprocket's — a small front ring paired with a large rear sprocket gives a low ratio, meaning each pedal turn moves the bike a shorter distance but takes less effort, ideal for climbing. A large front ring with a small rear sprocket gives a high ratio — more distance per pedal turn, more effort, better suited to flat ground at speed.
This is why a bike advertised as "21-speed" isn't automatically easier to climb on than one with fewer gears — what matters is whether its lowest available ratio is low enough for the hills actually being ridden, not how many total combinations the drivetrain offers.