Trying to understand Smart Contract Fees

Hello everyone, I'm starting to use some DeFi applications more and more recently and it's being a great experience so far.

As a part of DeFi applications, you have to pay fees to use them all the time, and some of them I just don't understand. I will list my questions below with some examples

Yesterday I used ALCX for some staking. At first, I had to "approve ALCX spend limit" on MetaMask and this transaction only costs me 0.00065315600004 Ether ($1.72) as shown here. My question is, what is Spend Limit Approval, and why fees for it are so low in comparison with other Ethereum uses cases? Second, I could only actually use the staking protocol after this approval shown above, why is that?

So I paid this fee and actually got able to use the staking protocol, when the fees were considerably higher. Why the fees for Smart Contracts are higher than a normal transaction? (this can also be seen on the EtherScan gas dashboard, where it shows that fees are higher for Uniswap Swap and Uniswap Add/Remove LP.)