Theres this famous drummer on YouTube named Copper who takes existing songs, removes the drums, and then plays live drums along with the other investments. How does he know what beats & rhythm to play for so many songs?
Is there any methodology to follow?
If you listen to a song, you will immediately be able to see what rhythm it is (4/4, 3/4 etc), what bpm, where the breaks are, how much swing is appropriate, where the accents are etc.
That would be the case whether or not existing drums were on the track. If there are no drums, you can pretty much fit what you like to the song, and a little experience will allow any drummer to do this.
If there are already drums, the skill lies in accentuating, rather than copying or just drumming around what is already there, but it is based on the same concepts:
So to summarise, he doesn't 'know' what beats to play, but experience allows him to feel where beats could work and he adds these in.
re "If you listen to a song, you will immediately be able to see what rhythm it is (4/4, 3/4 etc), what bpm, where the breaks are, how much swing is appropriate, where the accents are etc." - I'm not sure I agree with that, although the rest of that answer seems good to me.
If there is an existing drum track, that must give you a huge clue as to where to start. But having just given some rudimentary drum lessons, my student asked how I work out when to start the basic rhythm, so that the rhythm fits the song. This made me realise that what's intuitive to me isn't obvious to everyone ! And I remebered having to learn this for myself.
If you want to try such drumming, and it's not that clear to you, try this :
Tap along to the song, and count 1-2-3-4. Most people can naturally feel the rhythm of a tune and tap in time to it. Normally, people will tap on the bass and then snare strikes (just intuitively - eg if you listen to a crowd clapping along to music). You'll end up with 1-2-3-4 = bass-snare-bass-snare. Sometimes it doesn't quite work and you have to do this half speed for slower songs or double speed for faster songs like Ace of Spades.
On your drums, play a bass drum and a snare as above along with the song .. there's your basic beat.
Note that I'm assuming the rhythm of the song is "even" ie not 5/8 or something odd, where this method will fall apart. Most popular music is pretty evenly paced.
This is probably how a lot of people gain the ability to drum along to anything.
As of there - for the song you linked to, the bass drum riff is accentuated to be more funky, which is where the feel for the rhythme comes in - and to be honest the choice. You can try a few things decide what you think works best.
For fills etc : It looks to me like this fella is going a bit wild and putting in everything he can think of which probably relies on experience ('this fill fitted with this rhythm last time I did it on another song..'), "muscle memory" and a feel for what he wants to project.
In general, look for a phrase that's being repeated. This is the main "building block" for each part of a song. When you've identified the phrase you know when the bar starts and ends. Next, try to count within the bar. The best first guess, as others have indicated, is trying to count to 4 along with some notes played (e.g. notes accented by the bass guitar). If it fits, then most likely it's 4/4 and you can start figuring out where do accents go.
If it doesn't fit, then there are two possibilities. First is easy: it's 3/4 or 6/8 or something like that, so it's easy to deduce by counting that going to 4 is 1 off, and experimenting with counting to 3 (e.g. when it's 3/4 or 6/8) or 5 (e.g. 5/8) makes it right.
Second possibility is that the time signature is something like 5/8 or 13/16 etc. This is indicated when counting to 4 is off with the phrase, but not exactly by some integral number of 1s. For 9/8, for example, it's often easy to hear, that you're missing a half when counting to 4. Then you experiment with counting to 8 twice as fast (if possible) and count 9 notes. If the tempo is too fast to count with 8s, then you need to be accustomed with playing 9/8 - it's easy to imagine as ordinary 4/4 + 1/8 - just "one note twice as fast" added. What you can hear for sure is whether your counting too much, or too few. So you can just experiment with different signatures until you guess the right one.
Of course there may be even more complicated compositions, or parts of composition when it's not easy to hear a repeating phrase. In such case you can try focusing on accents.
