This might help give you a basic overview.

Internal Family Systems Therapy | Psychology Today

I have bpd and early on we worked with part theory even though he's psychodynamic/psychoanalytical. We didn't use those terms either though.

I always used to feel so fragmented and I believe it's trauma that causes this. I felt like I had 3 parts, which were very distinct. Two of which were children aged 4 and 7 and my current age. It was about identifying and giving space to allow each of them to talk. The youngest did nothing but cry because at that age I didn't really have words. Two years on I feel more whole. I actually feel each part less.

Similarly he said he was okay also look at inner child work when I brought it up. It might sound woo, but if you're interested I would really recommend the series Homecoming which is available on youtube by John Bradshaw.