No upscaling as it's not necessary. (But better players have better 1080p to 4k upscaling too, as the algorithms are more sophisticated, e.g. edge-adaptive scaling, temporal filtering, etc.)

First, the player performs MPEG-4 HEVC decoding, reconstructing full video frames from heavily compressed data.

Once decoded, the signal is still not in a display-ready format.

UHD Blu-rays are almost always encoded in 4:2:0 chroma subsampling, meaning luma (brightness) has full resolution, but chroma (color) is spatially reduced. so one of the first steps in the pipeline is chroma upsampling (chroma reconstruction). After that, the player applies color space conversion and output formatting, usually converting to a HDMI-friendly format like YCbCr 4:2:2 or 4:4:4.

HDR handling is sometimes done on the player. The tv is doing a last stage processing that is fine tuned for it's display like contrast enhancement.

I hope that helps

Most blu-rays are 1080p, not 4K. The latter gets marketed as "UHD" and sold in a black case, to contrast the blue case of traditional FHD blu-rays.