I know this has already been answered, but for those who are also python users, there is another and probably better option:
Then do a pip install on it as for any other python module.
I prefer this route as it avoids having multiple python versions loaded on my machine.
The standard install for the current version of pgadmin4 will download python along with it.
As far as other tools go, I prefer to design my database using a modeling tool.
As the only database we use or will use is postgresql, I find
https://pgmodeler.io to be pretty good for e free(ish) product.
It provides support for many postgres-only features such as extensions, functions, json/jsonb and so on
And for a new python user, the product manual (contribution required) is an excellent source for postgres-specific design topics.