Let me explain in simple terms. When it comes to rewards, POS is like POW but you just need to hold your coins in a wallet that can stake. In general, the rewards doesn't necessarily comes from transaction fees alone, rewards can also include new coins that is mined from staking and is added to the circulating supply. Though reward distribution method varies from different projects.
so aside from transaction fees and rewards from newly mined blocks, are there any other methods to generate these earnings? and do you happen to know what are the methods used in the projects i mentioned in my original post please?
Some projects do open investments, like ICO, where there are a lot of discount once you bought a certain coin before its official release. Hence, that is the best option/reason why other coins/tokens can give huge amount of rewards to its system's / token users. They can also have their own trading platform like Binance, which made their own token/crypto named BNB (Binance Coin). Hence, most project either DeFi or an ERC-20 project always starts with investment programs and stuffs to help the project make huge reward afterwards.