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This is called coin control and electrum has had this option for a long time now. On the addresses tab select the addresses you want to spend from. On PCs you can select multiple addresses by holding down the ctrl key when clicking on an address. Then right click and choose "Send from". There is no need to freeze addresses.
Brilliant, thanks. From what you write it sounds like its limited to address though and would not work with individual inputs, correct?
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I have read about BTC since your post "there are no "from addresses" even though blockchain explorers show it like that." And I understood that how it wokrs. Can you suggest a program or method to easily create transaction "by hand". Or link me to some informative place.
Very detailed information can be found here[1] and here[2], if you want to play around with TX created by hand I suggest bitcoin core. You dont have to fully sync it for this. Just use regtest or testnet (significantly small than mainnet). That is recommended anyway, because you dont have to risk any actual bitcoin. If you need testnet coins, just send me a PM with a testnet address and I will send you a few to play with. A QR code would be even better so I dont have to type the address

Electrum uses .txn files to import signed/unsigned transactions its mainly a wrapper around a raw TX though. You probably know the small window electrum open for transactions where you click "sign" and "broadcast" to actually send it. You can also click on "save" to create these .txn and get familiar with the format. Once you know how you create a raw TX with core and how electrum handles .txn file you should be able to do select individual inputs and not just addresses, but as I said above its overly complicated for the little amount of control you gain over the method Abdussamad mentioned.
[1]
https://bitcoin.org/en/developer-reference#createrawtransaction[2]
https://bitcoin.org/en/developer-examples#transaction-tutorial