I am afraid you are requesting a teeny bit too much asking for explanations about two very complex teachings, one of which is found in a highly controversial book in the NT, called the book of Revelation,which is filled with largely symbolic language that is never explained. There are flying dragons and scrolls, and whatnot, all of which defy gravity, both literal and symbolic.
This means that we as humans need to supply the meanings attached to those symbols, and naturally not everyone agrees with these explanations.
It cannot be covered in 5 minutes. Vanderhoven7's crisp and clear video above is an excellent summation of the Watchtower position and required 14 minutes to explain.
1. The concept of the 144,000 can be either symbolic or literal. Take your pick.
The Watchtower Leadership, who conceive themselves to be the elite spokesmen for God, believe that they have an explanation that is uniquely divine, and is not open to dissent or debate by the rank and file followers.
They believe and teach that the 144,000 are part symbolic [made up of symbolic Jews from 12 symbolic tribes of Israel] and part literal [the number from these symbolic tribes is somehow literal]
2. Some [like the SDAs and other Reformed Churches] believe that the 144k are symbolic in that they are spiritual Jews who are thus identified as the entire body of believing Christians who have been born again, and who have replaced the Jews as God's people.
3. Those like me who belong to Dispensational Churches, [I'm a Baptist] believe that they are literally Jews from the 12 "Tribes of Israel" as Rev 7:4 says. To the charge that these tribes no longer exist and thus cannot be the literal Tribes of Israel the reply is that the tribes are not actually non existent, it is simply that the records to prove who belong in these tribes are lost. There is nothing to suggest that these records cannot be archaeologically recovered.
It must be pointed out that the 144k are not presently in view, but will be manifest only AFTER some items of prophecy have been fulfilled. See Rev 7:1, where John says that he saw the 144k only "After these things" which is what is portrayed in the last verse of Rev 6, which appears to describe the start of the Great Tribulation. Thus the 144k are, according to the literalist view, a purely Great Tribulation phenomenon.
In all fairness, a Watchtower Follower once told me that "After" in Rev 7:1 is symbolic, and must be interpreted as "Before these things", that is, going all the way back to 33AD.