Information Architecture deals with how Information in SharePoint is presented to the End Users. The goal of an Information Architect is to design and present Information in a way that it is easily accessible to the end users. They should be able to search for the Items that they are looking for.
Navigation is one of the aspects of Information Architecture. The Architect can control the way how the user maneuvers over the site. Another point while considering developing a good architecture is that it should be easily managed. After the process of Installation and migration, the IT department has to be in charge of the Intranet and it would make their lives better if it is easily manageable.
Organizing content is the next part of Information Architecture. Some content maybe sensitive and needs to be placed in a specific location of a site and some content should be displayed publicly among all users. The Architect needs to work with the Compliance Team of the company in order to find the requirements regarding organization of documents.
Folders are a good way to store files and documents but with SharePoint 2016 this is slowly becoming a thing of the past. Now the trend is to use metadata to tag documents. Metadata radically improves the search capabilities of SharePoint. Once documents are tagged they would appear in the Search Results based on the tag. And through metadata there is the ability to connect or link disparate sites. For Example you might be in the HR site collection, but through metadata you could also get results from other site collections as well. And another good thing about metadata is it eliminates redundancy in terms of document creation. With metadata, you wont have documents in two different site collections with the same file names.
Information Architecture is all about creating or design a proper structure and then organizing the content in a way so that end users can easily access them.