I do not know how they will give the 7% monthly constantly, recently we had bitconnect that was giving fixed profits and crashed now, so we need to be little careful. on the other hand Eth is still working on the pow model they will introduce pos after few upgrades I guess.
You should learn of PoS. Proof of Stake. How that works. Search this forum or goggle it. Bitcoin uses PoW, Decred uses PoS. Noone pays anyone anything. New coins are getting generated during PoS and they are reward stakers get. Decred with PoS secures its blockchain.
Other way more legitimate question is. Is PoS a good way to secure network. There was and still is a debate of this. Ethereum will soon be biggest PoS experiment. There will be huge reward for the one that will break it. $107 billions currently.