About Kauthar...

It has been narrated on the authority of Hadrat Sa‘eed Bin Jubayr (May Allah be well pleased with him) who narrates on the authority of Hadrat Abdullah bin Abbas: He said about Kauthar that it is all that is good which Allah Ta’ala has granted to the Holy Prophet (Sallallahu Alaihi wa Sallam).

Abu Bashar says that that I asked Hadrat Sa‘eed Bin Jubair that people tell that it is a stream of Jannah. Hadrat Sa‘eed said: That is also a part of that good which Allah Ta’ala has granted to the Holy Prophet (Sallallahu Alaihi wa Sallam).
Hadrat Allama Ismail Haqqi (may Allah shower His mercy on him) writes in the exegesis of this verse of the Holy Quran:

What is more obvious is that all the bounties of Allah, be they outward or inward are included under Kauthar. The outward bounties are all the bounties of this world and the hereafter and the inward bounties mean all those sciences and divine knowledge (Ilm Ladunniya) which cannot be acquired by any senses but only by the grace of Almighty Allah. [Ruh al-Bayan, Vol. 10, Page 524]