It has huge data libraries with many feeds from many different sources
If you look at the Google Earth Engine data catalog (https://developers.google.com/earth-engine/datasets/) what you see are public data sets that are available from USGS, Amazon AWS and other sources. It could be that some of the data they provide is unique to Google, but I've not found any like that, so my guess is that over 98% of what they provide is the usual public domain stuff like Landsat, Modis, etc.
For example, here are the Sentinal-2 cloud optimized GeoTIFFs on Amazon AWS:Sentinel-2 Cloud-Optimized GeoTIFFs - Registry of Open Data on AWS (https://registry.opendata.aws/sentinel-2-l2a-cogs/)
What Google has done is republish public data in a way that is locked within their proprietary application so you are forced to rely on Google for everything you do, and to agree to Google surveillance of everything you do and every byte you touch, and to agree to let Google do whatever they want with what they learn about you and your work. You don't own your behavior on Google Earth Engine - Google owns that.
They know what you browse, what portions of the Earth interest you, whether you are interested in higher res imagery of a particular town, and they can easily infer what your politics are on climate and similar matters based on how you work with the data. They have very, very refined AI for that, and for data mining the associations between the vast wealth of data they have on you from all the other points of contact you have with Internet.
They can and do use and sell that information however they want. So don't be surprised if your YouTube recommendations suddenly start popping up videos or the ads Google shows you on other websites are based on inferences Google makes about your interests as derived from the data you looked at on GE. That's their business model. As they say in Las Vegas when people complain about losing a bit more than they planned at the tables, "Hey, somebody has to pay for all those lights..."
I'm not slamming Google, because it is a legitimate business model and they provide the convenience of collecting a variety of data in one place with a common API for writing code that accesses that data. They also are very good at citing the original source of the data, so it's clear they're not trying to take credit for, say, Landsat 8. They don't provide links to unrestricted access to that data, but based on Google's descriptions it's generally very easy to find such links, including to WMS and other web servers that provide open access to public data with no fees, no surveillance and no licensing restrictions.
I have yet to find a data set in the Google Earth Engine catalog for which I could not find the original data source (or sources that republish the data in unrestricted form) and that I could not use the data I wanted in Manifold. Release 9 is very good at being able to work with many different formats, etc. If you have some specific data you're interested in, say what it is and people here can help you find the original source and provide tips for using it.