Thank you for the question.

“White rot” is somewhat of a generic term referring to the type of wood that is undergoing decay in a tree. More specifically, when all wood components of a tree are affected due to fungal infection, it is termed “white rot”. The two major families of fungi that are causative agents are Basidiomycetes and Ascomycetes. See:

https://ipm.ucanr.edu/PMG/PESTNOTES/pn74109.html

https://forestpathology.org/general/wood-decay/

Specific mushroom identification is often difficult when only images are available. Microscopic examination is also needed. Having said that, my best guess for this specific mushroom (picture #3) is that it belongs to the family Ganodermataceae. Artist’s Conk (Ganoderma applanatum) has some visual similarity. It is a Basidiomycete so that would be consistent with its causing white rot. See:

http://www.minnesotaseasons.com/Fungi/Artists_Conk.html

https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2018.01557/full

The extent of actual rotting within the tree is difficult to determine from your pictures. Your next best step is to have a licensed, professional arborist make an on-site inspection and assessment. See:

https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional

Good luck. Thanks for consulting us.