the גדלות of the תנאים

there is a lot to try to understand when it comes to the גדלות of the תנאים,
i just learned about how reb akivah understood all the תגין of the letters in the torah, my question is, what does that even mean? can we understand it by example? i understand it that the torah is so deep and everything has meaning, but this (and other stuff too just saw this so i use it as example) for example, how should i understand what it means, they say that the משלים1000 that shlomo hamelech learned into the torah means that the פשוט פשט has a deeper meaning and this deeper meaning is the moshol on the פשוט פשט and that deeper meaning has even a deeper meaning and thats the moshol on the moshol and shlomo hamelech went 1000 times into it deeper and deeper,(btw is this the pshat?) so how can we people understand it even a little bit, or can we?

It means that the Torah understanding of the Tannaim (and not only the Tannaim) was supernatural. It was not astoundingly high IQs that gave them this level of knowledge but rather Siyata D'Shmaya and Ruach HaKodesh where they were granted knowledge that is impossible to obtain בדרך הטבע.

So later generations never had a chance to learn these things, or can those who deserve it learn it in olam habah? Can anything new be learnt in olam habah?

Yes.