Neither, it’s using anisotropic filtering. Anisotropic isn’t some additional feature that can be used along side Bilinear or Trilinear, it’s the next filtering type after those two options. Unity’s texture options kind of hide that fact as you can’t choose “Anisotropic” in the same dropdown, but on the graphics API side there’s a setting for how to filter a texture during “minification” (when it’s being displayed smaller than the actual resolution), and the options are Point, Bilinear, Trilinear, and Anisotropic.
Anisotropic filtering is basically trilinear on steroids though.
Point filtering just samples the single nearest pixel on the smallest mip appropriate.
Bilinear filtering samples four pixels of the mip and linearly interpolates between each pixel pair, and the interpolated values of those pairs.
Trilinear filtering takes billinear filtered samples from both the smallest mip appropriate and the next mip after and linearly interpolates between them.
Anisotropic filtering takes multiple offset trilinear filtered samples using a larger mip than the other filtering modes would use. The number of samples depends on the angle of the surface and the max anisotropic level set, and the specific implementation used by the GPU. All of the consumer GPUs are using approximations to reduce the number of anisotropic samples needed, but even with out that if you have 16x aniso set it might only be doing straight trilinear on many surfaces like those facing directly at the camera, it’s only those that are nearly edge on that’ll ever get the full 16x samples.