Grab one of the lenses off of your DSLR and hold it up to your camera lens… backwards.

Like, look through the objective and out the end that normally hooks up to your camera. You may find that you’ll have to manually focus, and your effective depth of field will be comically short. However.

You will probably also find that this grants you a fair degree of magnification, probably more than your phone can muster by itself, and it also allows you to focus on subjects so close that they’re almost touching the lens. Here for example is the marquee on a jelly bean, resting atop my grungy mousepad. I took this through the kit lens that came with my R10, backwards, pressed up against my phone’s little pinhole camera. It’s a tad overexposed. You get the idea.

Ideally you’ll want the dinkiest, lowest tech lens in your collection for this. My Canon RF glass is actually a rather poor choice because the lens assemblies are completely electronically controlled by the camera body so you can’t twiddle the focus or aperture and in my case the lens is just left floating at whatever settings were last used on it. If you have an oldschool all manual lens you can tweak those settings at will (and probably wind up with a lot less vignetting than I did).

You can also by and large press a wide array of random cheap prime lenses into macro duty by way of attaching a cheap extension tube between your lens and your camera. True Jedi back in the day I suppose had to cobble together their own via various gimcrack arrangements involving cardboard tubes and tape, but nowadays the Chinese will sell you a perfectly functional extension ring that even passes through your camera’s lens control pins for a couple of bucks. I have a set I got from Jeff Bezos’ Big Rock Candy Mountain for about $20, and while I wouldn’t be in a rush to dangle my $1250 RF100 F2.8 off of the end of the stack of them while, say, hanging out of a helicopter, for stationary macro use they’re just fine.