Hi The Toddler,
There is a difference between a mattress topper, a mattress pad, and a mattress protector although there is some overlap in function between them. In most cases you wouldn’t use a mattress pad and a topper together (although it’s certainly an option that some people may choose for specific reasons) and there are no “rules” but in most cases a foam mattress pad will go directly on top of the mattress and then a mattress protector would be used on top of that and then your sheets and bedding would be on top of the protector. There is more about mattress pads in post #10 here (and in some cases some mattress pads can perform a dual function of a mattress pad and a mattress protector) and there is more about mattress protectors in post #89 here.
In some cases such as a wool topper where someone wants more of the ventilation and temperature regulation properties of sleeping more directly on the wool they may choose to use the protector directly over the mattress (to protect the mattress but not the wool topper) and then use the wool topper over that (and then their sheets and bedding) so that the mattress protector doesn’t interfere with the properties of the wool topper.
I would focus more on the materials and design of a mattress topper, mattress pad, or mattress protector than the brand because the name on label means much less than the specific materials or design of the product. There would probably be almost as many preferences as there are people but some of the more popular options (out of many hundreds of similar choices in a constantly changing market) are included in the two posts I linked that can help you with “how” to choose between them.
Phoenix