Both use peroxide chemistry. The differences are concentration, how well the gel stays on the teeth and off the gums, and whether anyone checked your mouth first.
Custom trays hold gel against teeth and away from gums. Strips and stock trays cannot match that.
Concentration
Professional systems generally use stronger formulations, used under guidance.
Assessment first
Whitening over untreated decay or exposed roots invites problems.
Sensitivity management
Concentration and wear time can be adjusted for you.
Expectation setting
Someone tells you beforehand what will not change — including your restorations.
When a store product is reasonable
For mild surface staining on healthy teeth with no restorations at the front, an over-the-counter product may be perfectly reasonable. The situations where it disappoints are uneven results, existing dental work, sensitivity, and internal discolouration.
If you have tried a store product without result, that is useful information rather than a reason to buy a stronger one.
What we can explain here — and what needs an exam
A page can inform you. It cannot examine you.
We can explain online
Why tray fit matters
What supervision adds
When store products are reasonable
Determined after an exam
Whether your teeth will respond
Whether anything needs treating first
Which approach suits your sensitivity
What to do next
If a store product has not worked, bring it up — the reason is usually diagnosable.