Thank you. I'll try that.
So, one more question that comes to mind is what happens if my phone gets stolen and hacked?
Let's say it does, but in the meantime I use a second phone to restore my wallet using the exported extended private key and 12 words, I should see the same funds, right?
But so does the one who hacked my phone. What does one do in this case?

a PIN code should be required to send the funds so whoever stole the phone, won't be able to take the funds. In the meantime, you can send your funds to another wallet you have so even If he manage to get the PIN afterwards, It would be useless because you sent the money somewhere and Its empty now.
Great! thanks for the tip.
I was thinking of using the Local Trader in mycelium and read the guide:
https://mycelium.com/lt/help.html I then followed to the description of what is HD wallets.
Just to be really sure: All I need to do once I set up my 1st wallet is to back up the 12 words (master seed) by writing it down and also export the private extended key, and do it only once, not after every transaction or X days, correct?
So if in 2 years time I gained X bitcoins and I need to restore my wallet using master seed + extended private key, it will restore it with all the funds?