After going through 25B AIT, how prepared are most people for certification exams? Do we get any certifications while in AIT?

Currently, you don't get any industry certifications through the 25B course.

Your preparation level depends completely on you. You can leave AIT ready to take your Security+ and CCENT exams, but you'll have to study on your own after class.

I certified in telecom before they retired my MOS-26C. It's like 25B but more towards maintenance. I still had to buy the Exam-CRAM kit to self study, and then pay for the exam using GI-bill. Personally haven't found as much value in certs as with my bachelor degree also paid by GI-bill. Save your cash and use your benefits first. Many vets have GI-benefits but don't use them.