If you are using Esri software, you can access their curated library of geospatial data products contained in The Living Atlas.
You can browse the contents through the web link (above) or search for layers directly while using the Map Viewer tool in ArcGIS Online.
The Living Atlas contains data from government sources (such as the US Census) and other researchers already formatted for use in ArcGIS.
Other Esri products, such as Business Analyst, contain even more detailed financial and demographic data and may be worth exploring.
Many other types of data can be integrated into a GIS if they have some kind of geographic information included in the dataset.
Listed below are some resources for finding data that is mostly well-formatted to easily upload to a GIS.
The Working with geospatial data section provides more detailed information about how to integrate table data and your own field data into GIS.
Los Angeles Area
Open Street Map is a crowdsourced mapping platform that includes open source geospatial data contributed from users around the world.
