Not sure if this method is no longer functional because this package[1] is not maintained anymore but from the guides that I found online (some recent ones) and also past experiences, this should work:
sudo add-apt-repository ppa:bitcoin/bitcoin
sudo apt-get update
sudo apt-get install bitcoind
The last command is returning this error:
E: Unable to locate package bitcoind
For bitcoin-qt, I ended up installing it from the Snap store (because I was getting the above error as well) but I can't seem to find an alternative way to install bitcoin daemon?
[1]
https://launchpad.net/~bitcoin/+archive/ubuntu/bitcoin
