I am going to offer my view on your questions and concerns.
1. So with Ledger, for instance, it’s a “24-word recovery phrase.” I was wondering when and how the recovery phrase was determined on a Ledger device for example? It’s not possible that the company itself knows it before they deliver it with DHL?
The recovery phrase is generated on your local machine during the setup process. Ledger doesn't do it for you. Still, these are closed-source devices and you have to trust them that they don't have backdoors in there somewhere.
I’ve heard the best way to buy a Ledger is from the company itself, because Amazon/eBay Ledgers could be compromised.
That's true for all hardware wallets, not just Ledger. I am not sure what you mean with compromised. Only a genuine Ledger can connect to the genuine Ledger Live native app and communicate with genuine Ledger servers. If you use a fake HW with a software downloaded from a phishing site, that's not Ledger's fault.
2. Just these 24 words in the right order are pretty much secure?
More than secure. The security of the 24-seed phrase is higher than that of a bitcoin private key. Don't worry about someone generating the same seed as you. Worry about all the planets in the solar system come crashing into planet Earth at the same time and you being the sole survivor. That's more likely to happen.
3. The 24-word recovery phrase in universally compatible (whether other hardware wallets, software etc), so if a thief knows my recovery phrase, he can also use it with Electrum obviously?
Yes to both questions.
4. And this 24-word recovery phrase is able to generate thousands of public/private keys, so if I have a Bitcoin savings plan like 0.1 Bitcoin every month (so sending it from Coinbase/Binance to the hardware wallet), nobody would find out about the real balance since it’s spread out over many addresses?
The servers you connect to can see your addresses and the balances on them. That's true for Ledger Live as well as Electrum. The way around that is operating your own server. The information they have on you impacts your privacy but not the security of your coins.
5. If I go for a Ledger, I would probably purchase the Nano S Plus because it doesn’t have any bluetooth?
That's not a question. If I wanted to buy a new Ledger device today (which I don't), I would rather get the Nano S Plus than the Nano X. I wasn't a fan of the Bluetooth feature the 1st time I saw it and that hasn't changed. Their battery system is also bad.
6. I can setup a Ledger also completely offline with Linux/Ubuntu DVD and just Electrum?
Ledger ships its devices with already installed firmware, but that firmware could be outdated. You need internet connection to get the latest firmware. You also need to install the apps you intend to use. You can only do that through Ledger Live. You can't use Electrum and Bitcoin without first installing the Bitcoin app on your Nano.
7. I’m also concerned about the Ledger since there was this data breach online, so the thing to do would be using the address of a homeless shelter, something like that just to stay anonymous?
Or you could try to ship it to a PO box if that works in your country and Ledger supports it. Additionally, you might try your place of work or something like that.
8. If I have everything setup with Ledger Live or Electrum, can I have a watch only wallet just for transferring my Bitcoin from Binance/Coinbase with a new public address even if I chose do destroy my Ledger device after 1 week for additional safety?
I don't see the point in destroying the device. Don't buy it at all if you aren't going to use it as intended. Look into setting up and using an airgapped system instead.
9. So the Ledger device in general is pretty much 100% secure and the only known hacks and 100% of the time it was the user who put the phrase up on Google drive, told their partner etc.?
No hardware wallet is 100% secure. Something can be less or more secure, but not completely 100% secure. A (Ledger) hardware wallet is secure enough if you don't mind its closed-source nature.
10. As far as my understanding goes, there are only 2 ways to “hack” (not the right word of course) a Ledger device: 1. someone knows 24 word seed or 2. Ledger device and someone knows pin code. That’s correct?
In theory, each device could have a backdoor that reports the generated seed phrase back to someone at Ledger. No one knows.