Your 1st 3 stops should be Ubuntu packages, Launchpad and the official website.
Version 11 is only available in 19.04 and the package name and installation command is:
sudo apt install postgresql-11
$ sudo apt install postgresql-11
[sudo] password for rinzwind:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
libllvm7 libpq5 postgresql-client-11 postgresql-client-common
postgresql-doc-11 libjson-perl isag
The following NEW packages will be installed:
libllvm7 libpq5 postgresql-11 postgresql-client-11 postgresql-client-common
0 upgraded, 7 newly installed, 0 to remove and 31 not upgraded.
Need to get 27,4 MB of archives.
After this operation, 104 MB of additional disk space will be used.
Do you want to continue? [Y/n]
if you want it in 18.04 you can also download it from the 4th link above but you will need to check all the dependencies.