I also recommend to you to use electrum mobile wallet you can save your bitcoin there and don't forget to save the 12 words phrase seed
Because 12 word seed phrase is your backup to recover your wallet in other wallet or you can recover it in your desktop electrum wallet..
So if your phone was loss you can be still recover your bitcoin that was save in your mobile phone..
And you can recover it also in other wallet just save your seed phrase in piece of paper or notepad anywhere in your computer..
Same ask is this words Fixed or can be changed ?
I think for electrum mobile wallet its a fixed words phrase if you create account in mobile you should save this seed and it will ask again to manually type the 12 words seed..
It can be change if you create new wallet but your old wallet will be change too if you make a new wallet created using electrum mobile..