How do retainers prevent underbite from relapsing after ortho? I know they prevent movement within the arch but how do they prevent bad bite relapse?

Orthodontics. If you've been treated adequately by a skilled Orthodontist, he/she can discuss these issues with you better on a one to one basis, rather than a simple answer over the internet. If you have not seen an Orthodontist, find one and discuss treatment options, prognosis, limitations and retention afterwards.
Good Question . Conventional retainers and aligners cannot prevent relapse caused by additional jaw growth. I always prescribe a Tooth Positioner which if worn properly will re- direct the growth and keep or even improve the correction. There are several companies that make positioners, your orthodontist will be able to supply one. Remember- it only works if you wear it! Good luck.
Secure bite. Orthodontic treatment should establish a stable bite and correct various malocclusions(improper bites). It does straighten teeth for better alignment and looks but much more. Once corrected, the retainers will hold teeth in position and secure bite corrections when worn as directed. Some type of retainer or appliance should be worn thru life. .
Lifetime Commitment. The natural tendency and long memory for teeth to return to their original position is the chief reason why retainers are necessary to prevent bad under-bite relapse. Your success based on how faithfully you wear the retainers. It is a lifetime commitment.
Depends on cause. If the cause of your underbite was skeletal, at 28 you are no longer growing. If the cause was dental, your retainers will help maintain your teeth in their new relationships as long as they are worn as directed. Follow your Orthodontic Specialist's instructions to the letter.