When resizing or rotating a video in Adobe Premiere, what interpolation method is being used by default? Bilinear, Bicubic, Neares Neighbor, Lanczos? I know using Transform effect we can chose between bilinear and bicubic. Any ideas? Thanks

I once made a feature request for user control over scaling algorithm but nobody cared lol.

The interpolation method Premiere Pro uses depends on your renderer setting, which you can check in File > Project Settings > General.
 

By default, Premiere Pro uses the GPU renderer, which means scaling and rotation use the Lanczos 2 low-pass sampled with bicubic algorithm. This is the best all-round scaling algorithm, and because it runs on the GPU it remains fast despite its quality.
 

If you have switched to the Software renderer, the algorithm depends on whether Max Render Quality is enabled at export:

• Max Render Quality off: Premiere Pro uses Gaussian low-pass sampled with bilinear, which is fast but produces slightly softer results.
• Max Render Quality on: Premiere Pro uses Variable-radius bicubic, which is slower but sharper, similar to standard bicubic in Photoshop.
   

For a deeper dive into how these algorithms work, Jarle Leirpoll has an excellent article on the topic.