Thanks everyone.

Does the expression"an arm of,an ocean of,a sea of,a multitude of,many,if not most"have the same meaning as "many"? Yes And does the expression"flood/swarm/slice/pour attention into" mean the same to "pay attention to"? No. It means overwhelm/overrun or swamp or invade

You have to think figuratively. The problem seems to be that you're expecting a literal explanation. For example, "as soon as we set up our picnic, an army of birds appeared". The idea here is that the many birds were LIKE an army, and would probably "invade" your picnic.

Thanks all.I think some material books have misleading me

No, they don't have the same meaning. Have you tried a dictionary?