First the how
There are several methods of affixing a filter to your smartphone, possibly more than this article will show. These are the methods that I use for pairing filters with my smartphone.
1. The Universal Lens Clip.
The Universal Lens Clip is available to purchase for just a few dollars, also from Wish with an accessory lens for smartphones. This clip has a 37mm thread onto which those accessory lenses fit. It's also sold with a filter set, again on Wish and again the filter size is 37mm. If the clip is as much as you want to use, for some it'll be the only sensible option, then building a collection of filters at 37mm is all you need.
My own set of filters are 55mm, you'll see why later. Obviously, 55 doesn't fit directly on 37, so a step ring is required. Fortunately, there is such a thing as a 37 to 55mm step ring, thus allowing me to use my filters with this clip.
2. The traditional method, filter attached to the lens.
We smartphone photographers have quite a range of accessory lenses that we could purchase, Moment and Struman Optics are just two. Of these options, I personally use the latter and for two of my smartphones have Struman Optics phone cases, my current phone is not supported, hence the Universal clip.
Struman Optics offer a Circular Polarizing Filter (CPL) to use with a couple of their lenses, the Cinematic Wide-angle and Cinematic Portrait. This filter is 55mm, a standard fitting and that there is an extra ring in the kit. This ring is an adapter that fits the Cinematic lens to allow the use of the CPL. It follows then, that any 55mm filter can be fitted or, with an additional step ring, a larger filter if you have any from your film photography days..... I am that old!
I have a Cinematic Wide-angle and the filter kit, this is why I have chosen the 55mm filter path. Had I chosen 37mm then the filters would not work with the Cinematic lens.
Another filter management system is the NiSi P1 Prosories Mobile Phone Filter Kit. Options for this square-shaped glass are a polarizer, graduated neutral density (ND) filter, 3-stop ND filter, a blue light night filter. Click/tap to read more here.
3. A Struman Optics case and Struman Optics lens clip.
Whilst the range of cases that Struman Optics sells is increasing year by year, the range is still limited. For my previous daily use smartphones, I have purchased a phone case from Struman Optics to fit my Struman lenses too, Unfortunately, there is no case my latest smartphone nor likely to be, so using a clip is currently the only way to use either lens or filters.
For those of you who can use a case then it is also possible to attach filters without using a lens. The thread on the case is 17mm, we, therefore, need to get up to 55mm. You'll need 55mm filters if you are going to use a Cinematic lens.
Non-standard step rings are nigh on impossible to find in Australia, I've scoured the internet and the best supplier is in the US. Even then there is no single item that solves this issue and the solution is two-step rings. A 17mm to 37mm plus a 37mm to 55mm. It's no big deal and they are cheap enough.
The lenses from Struman Optics will be supplied with a clip, not a Universal Lens Clip as described above but specifically for Struman Optics lenses. As with the phone case, the thread is 17mm therefore the same method as with the case can be used to mount a filter only option.
So far we've seen how to use screw mount filters, arguably more convenient than resin filters but the resins are better for their graduated filters. Whilst a more cumbersome approach ( NiSi make nice, neat small filters but have a very limited range) it can be done.