Open Electrum Android App in READ-ONLY mode?

[_act_]

It will only look like a wallet that you can spend from but it is not if you use use the public key. The private key or the seed phrase allows you to spend, but the public key makes it a watch only wallet. If you do what OmegaStarScream said, it is a watch only wallet.

The master public key will start from zpub.

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[nc50lc]

After creating a standard wallet on Android and scanning the QR-Code from Windows Screen the wallet is created.
And it is writeable :-((((
I can send BTCs.

The 'Send' tab will only allow you to create a PSBT without any signature, it wont continue to broadcast an unsigned transaction.

In Android, you can check the wallet's type by tapping: "<Wallet name> -> Wallet details".
You should see a "Watch only" tag with an 'eye icon' above the screen below 'Wallet details'.

[_act_]

In Android, you can check the wallet's type by tapping: "<Wallet name> -> Wallet details".
You should see a "Watch only" tag with an 'eye icon' above the screen below 'Wallet details'.

If he open electrum, he will see the eye icon at the superiolateral side.

[nc50lc]

In Android, you can check the wallet's type by tapping: "<Wallet name> -> Wallet details".
You should see a "Watch only" tag with an 'eye icon' above the screen below 'Wallet details'.

If he open electrum, he will see the eye icon at the superiolateral side.

He already saw the main screen and still thought that it's not watch only.
Reading the literal "watch only" text should get him at ease that it's actual watching-only wallet.

[o_e_l_e_o]

I can send BTCs.

You can't, if you've followed the instructions correctly.

On Windows, click Wallet -> Information. The window which pops up will have a box titled "Master Public Key" at the bottom. Within will be a master public key starting with "zpub" (or maybe "xpub" or "ypub", depending on the specifics of your wallet). Use that key (and only that key) to create your watch only wallet on Android.

If you've done this, you cannot spend from your Android wallet. It will let you enter an address, enter an amount, set a fee, and actually create a transaction, but it will be unable to sign or broadcast that transaction. Instead, it will tell you to present that transaction to the "signing device". If you don't do that, then the transaction cannot be signed and your coins cannot be spent.

[Pmalek]

This is exactly what I do NOT want. This procedure creates/replicates a full wallet.

It replicates a full wallet to some extent, and that's the whole point of it, but it's missing the essentials to sign transactions. A watch-only wallet created with public keys or a master public key can't sign any transactions unless you made a mistake and imported the private keys/seed instead. You seem to be struggling with the terminology and understanding of how transactions work, so such a mistake is possible. The wallet you created can't know your keys unless you provided it to it. It's that simple.