The biggest downside to most of this stuff, especially for someone that's new to the SDR-based way of doing things 'cause it sure as hell ain't nothing like just punching a few buttons on a scanner to get something working, is that it actually does require some effort. The other problem is there's a bunch of separate applications/programs/tools that all need to be set up and functional on their own before you can even get to the point of trying to make them work with each other in tandem to accomplish just the one task: tuning in a radio signal that is either carrying analog (easy) or digital (the tough nut to crack) content which needs to be decoded for "the good stuff."
It's not impossible, no, just daunting and yet another problem crops up: there's no one single source for the info needed to get all of it working together. Sure, if you do a search here or using any major search engine for "sdr p25 decoding" or something similar you're going to find a bunch of guides written by a variety of people using a variety of software applications and programs and a variety of SDR hardware that somehow magically ends up getting the actual task of actually allowing you to monitor communications. It's a true miracle most of this stuff works at all, sometimes.
Having said that the basic workflow with the hardware and software you've got would be like this using the concept of a signal pathway:
Antenna attached to the SDR device (in your case, Airspy) for receiving a signal feeds the signal to Airspy which is controlled by an SDR application/program to tune in a given signal on a given frequency (or frequencies given the working bandwidth of such hardware) using a given modulation which is then fed from the SDR software application as either:
Analog (which can be and typically is filtered and not a baseband signal) in which case it's sent directly to the sound card for playback through the computer's sound system
or...
Digital which is passed from the SDR application/program (unfiltered basedband for the raw signal) to DSD+ (or the original DSD as some folks like to use that even nowadays) by traditionally using a virtual audio cable (so there's no audio processing done on the signal by the Windows audio mixer/EQ/etc) where DSD+ will be able to decode the digital format/protocol and provide a final product of workable intelligible audio a human can understand
Now that's just for straight analog or digital using conventional transmissions on single frequencies. Toss in trunked systems and it gets a bit more complex when using SDR setups:
If it's an analog trunked system, Unitrunker alone can be used to handle all the popular formats available today by decoding the control channel from one frequency and then tuning voice channels as required. It's more efficient to use two SDR devices (unless you own one like Airspy that has a wide enough bandwidth window to receive all the trunking frequencies at the same time as most tend to fall within a ~5 to ~10 MHz window). In either case, you have Unitrunker handling the decode of the control channel and also tuning in voice channels as assigned given talkgroups/priorities/etc. It works rather well, actually, but there's a learning curve as already mentioned and no seriously great documentation for it although there's a user guide that covers some of the most relevant stuff.
If it's a digital trunked system using any of the popular formats/protocols then Unitrunker can handle the control channel decoding for all the ones in use today
but obviously it can't decode the voice traffic which is where again DSD+ comes into play. If it's a P25 Phase I based system - I mention that because it's the most popular format/protocol in use worldwide - DSD+ can handle it easily. There are no software-based decoders
yet for Windows that can decode P25 Phase II but we're all hoping that the developers of DSD+ may add it in some future update if possible. If you actually need to decode a P25 Phase II system for voice traffic, assuming it's not encrypted, there's OP25 but that's a Linux-based GNURadio-based software application and somewhat "labor intensive" to get installed properly as it does require some compiling and command line magic so most folks won't go that route unless they have absolutely no other alternatives i.e. a physical scanner capable of P25 Phase II decoding.
But with Unitrunker + DSD+ you can readily monitor most any non-encrypted trunked system in use today given the lack of P25 Phase II voice decoding - Unitrunker can decode a P25 Phase II control channel so at least you'd be able to see what's going on in terms of talkgroup activity/radio IDs, but you won't be able to listen in because of the lack of DSD+ support for P25 Phase II.
I know, I know, more babbling on my part, maybe someday I should just write a damned book on all this stuff and make it dead stupid simple for anybody to get it working. There's two books about RTL-SDR hardware out now, one of them is not quite so easy to grasp for a newcomer to SDR and the other is just a collection of a bunch of articles compiled from postings at the RTL-SDR.com website so it too can be a bit tough to get a handle on since the language isn't tailored for newcomers: both books work on the assumption that someone has at least some basic understanding of radio and communications technology but they're worth considering I suppose.
My advice is do things in steps: start with the simple stuff (literally) and verify your Airspy or whatever SDR hardware device you're using is working as designed, testing it out with a variety of signals and modulations and learning what you can about the SDR application or program you're using to control Airspy. Then start adding additional capabilities by using Unitrunker, DSD+, virtual audio cables, and so on.
If what you're aiming for is a P25 Phase I system, you can do that with Airspy + Unitrunker + DSD+ easier than you might think at this point. In fact, when you do get it working (which you will as long as you don't give up on it because of frustration - been there, done that, almost planted one of my RTL sticks in a wall over it) you'll get that "AHA!" moment when everything comes together and you hear what you've been trying to listen to for what seems to have been an eternity of tinkering around with all of the patchwork stuff. It'll be worth it when that moment comes.