A page alias is an extra path value for a page.
If a page has an alias that corresponds to the path of a missing page, a redirection is performed.
You can see them as symlink or shortcut for a file system.
With the permanent page url, this feature is in a deprecation state.
Why?
Because when moving heavily the files, you end up with a lot of aliases and it becomes really difficult to manage them. The introduction of the permanent page url permits moving the file easily from location without this burden because it's identified by the unique page id
A redirect alias is an alias that will perform a permanent redirection.
This type of redirect are automatically created:
For instance:
A synonym alias is an alias that will perform a transparent redirection. The page will be served normally as if it was also existing to the synonym path.
You can create, delete and see the aliases for a page with the metadata manager.
You can query the aliases with the following statements:
select * from page_aliases_vw;
select
p.path as page_path,
pa.path as alias_path,
pa.type as alias_type
from
page_aliases pa
inner join pages p on pa.UUID = p.UUID;