The easiest way is to download google earth, and then use the historic imagery slider to find imagery that matches that used by onX.
There are a couple imagery services out that that basically all web mapping applications utilize. These are tile services, depending on the scale the imagery comes from different sources. At the highest most zoomed out level is satellite imagery (eg federal satellites) at the most zoomed in level the highest quality imagery is either from drones/ planes (eg.private company or imagery shot by a county). All of these layers are stitched together by companies like digital globe who consolidate the imagery and then package it. OnX buys these imagery services like anyone else, they have zero control over how often it's updated, clouds that distort certain areas etc. They have read only access to a stream, that they add to their map. If their imagery appears better or worse then bing/google/esri it's because they pay for a different level subscription or get their data from a different third party vendor.
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As far a parcel data goes OnX has really done an amazing job stitching together the country. There are a couple of other companies that have similar layers (ex Whitestar), but as far a nationwide layer OnX is probably the best option on the market. That being said OnX is stitching together data, my assumption is that they are either working with conglomerators like Whitestar or have contracts with individual counties (probably a mixture of both). Some counties will provide them a API that automatically updates their server on a regular basis, some counties might send them monthly or bi annual dumps, and some counties (and I know this from personal experience) only sell their parcel layers ad hoc and mail you a CD or thumb drive that comes with a shapefile and then an access database and it's up to you to join the two and parse out what information you want for each parcel. So, for some folks it might seem like OnX updates are happening in real time, but in other areas it might seem like their hasn't been an update in 5 years. All this kinda depends on the agency (county/parish/etc) or company that is collecting the data, there are a number of counties who are still solely using paper maps and therefore have literally no digital coverage.
Another complicating factor is data licensing, those contracts run the gamut. OnX seems to get around it by only providing the parcel shape and a small subset of the data, and not allowing you to download anything. Point being your county may have specific terms and conditions about how often OnX can get data from them and/or how it can be used.
Data comes in more or less three pieces the shape, the projection, and the attribute table. OnX has to take data from all these sources, put them in the same projection, standardize the table, and then append the data. The difficulty of this task greatly depends on what condition the data is in when you get it, some of it likely is pretty clean and just has to be loaded in a database, some of it is probably a dumpster fire and takes a lot of effort to correct before it can be loaded.
I'm a GIS Professional and just trying to give some context of how this all works, this may or may not be exactly what OnX is doing.