For example connected MetaMask to Revoke Cash security utility, when venturing into established/reputable De-Fi sites like Uniswap via your MetaMask wallet and signing/confirming transactions shown on screen in metamask, why is it by default for every transaction that the approved limit is set to Unlimited/Max and to revoke this it cost gas fees 
If I do not revoke and do not pay the gas fee to revoke past smart contracts then I am open to smart contract risks/hacks correct?
If I do not revoke and do not pay the gas fee to revoke past smart contracts then I am open to smart contract risks/hacks correct?My suggestion is that you never set the limit to unlimited, unless you know what you're doing, for example if you're going to make several transactions in a short period of time, want to save fees and then revoke.
If there is an exploit, they can use your approved limit to drain your funds, so you're correct
When transacting on De-Fi sites how do I set by default for the every Approved amount Limit to be auto adjusted to the amounts I want to just deal with? For example if I want to swap $100 USDC into $100 USDT on Uniswap then how do I auto set it to approved limit of $100 only not max/unlimited because in future I might forget about this past swap and lets say for example in future I deposited $1million stablecoins into Metamask and forgot to revoke the unlimited/max amount approval for the past $100 swap I did. Because I forgotten I risk losing $1Million 
?
I don't want to keep on paying gas fees after every de-fi transactions to revoke past no longer needed smart contracts.
If I do not revoke past smart contracts approval amounts then this is still open and can be used for future transactions on the De-Fi site meaning if I do revoke past contracts then I have to pay gas fees again to set up a new smart contract approvals amounts on the same de-fi site like uniswap?
Also what are Signatures in Revoke.Cash?
?I don't want to keep on paying gas fees after every de-fi transactions to revoke past no longer needed smart contracts.
If I do not revoke past smart contracts approval amounts then this is still open and can be used for future transactions on the De-Fi site meaning if I do revoke past contracts then I have to pay gas fees again to set up a new smart contract approvals amounts on the same de-fi site like uniswap?
Also what are Signatures in Revoke.Cash?
The site you're operating on sets the suggestion limit, so I don't know if you can just set it to the required value by default.
Here I always adjust it manually when they set it to unlimited
As I said, if you leave the limit at unlimited, as long as the contract exists you will be subject to a hack that could drain your funds. It's not easy, but sometimes it happens
And yes, every time you change the limit, or revoke it, you have to make a transaction and pay the fees for it
There are some networks like Zksync that do something different, where you can just sign to make the transactions, but most operate in the same way
Being practical here, I do a lot of transactions on a regular basis, and the only network I have to manage better is ETH itself, because sometimes it's expensive to keep approving and revoking them
The rest, such as Base, Arb, Op, Polygon etc generally cost 1 cent for the limit and revoke, so I don't even bother