Δεν έχω καταλάβει τι σχέση έχει η Revolut με το Ledger.

Και μένα εντύπωση κάνει. Σαν την paypal δε λειτουργεί κι αυτή; Δεν είναι crypto wallet.

This is a paid feature so it's not sending your seed phrase anywhere unless you pay $9.99 per month for it (which is a dumb subscription).

Δεν συμφέρει η PayPal.

Στείλτα σε τράπεζα (κατά προτίμηση Revolut που έχει 0 fees + SEPA instant).

When you buy them, there's the spread and that's considered by many as the fees. As for the transfer, it depends to the exchange or platform where you'll be doing that.
Some may have a bit of fee and some are free to do so but don't you worry, when you withdraw that from any platform, it's not going to cost you that much.
But the problem today is that the mempool is quite clogged and expect a couple of bucks could be the calculation for the transfer fees.

Ledger Live seems to be using Moonpay[1] so I would imagine that they might be charging users a bit more.

As for your second question, then any decent exchange should allow you to withdraw your funds to your ledger or any other wallet. And from what I understood from your last post, that should also include Revolut. Now whether they charge more or fewer fees than other exchanges, I'm not sure.

In addition to Kraken, you should also look into Coinbase.

[1] https://www.ledger.com/academy/series/ledger-how-tos/how-to-buy-crypto-through-ledger-live

If you are storing your cryptocurrency funds on Revolut or any kind of exchange/platform, you do not technically control and own the cryptocurrencies. In order to store for large amounts and long term it is recommended to move your funds into the wallet that you control.

Having a hardware wallet, Ledger, in your case surely helped you to ease the burden of maintaining secure devices for the funds. I suggest you utilize it fully. Also, if you need resources to learn deeper about cryptocurrency and how to store it safely, Ledger has a wonderful source you can take a look at: https://www.ledger.com/academy.

Can you withdraw your crypto is what I'm asking. if you can withdraw or send your crypto to your personal wallet, then I don't see any harm in using them. But if they don't, then you should switch to a proper exchange instead. Sell your crypto to fiat, and use it to buy the coin you want in Coinbase for example, and then withdraw that to your personal wallet.

Can you withdraw the funds from the account? If you can only buy/sell and transfer them to other Revolut users, then no, you shouldn't keep buying from there. Instead, use an exchange, and withdraw to your personal wallet.