Thanks for bringing this up, maybe this will be useful.

I don't practice either, but
>Dharana (concentrating on a single thought for a prolonged period without ANY mental interruptions)
depends on what you mean by concentrating and "interruptions". I can hold a single though in focus such as "I am" and repeat it now and then without anything else entering the focus, but I am aware of other thoughts trying to enter from the edges of my mind, I just don't have to react on them.

But I don't actively practice this, I don't meditate or do magical workings by focusing on a thought. I do breathing exercises, chants or mudras and focus on my bodily position, or I read a text and focus on the text, or when doing calligraphy I focus entirely on the motions.

I can hold a thought for a minute though if I try, didn't attempt to go longer, but I may pick this up as it can be a useful skill.

>Dhyana (emptying the mind of all thoughts for a prolonged period without ANY mental interruptions)
If this means absolutely nothing even tries to get in, or enters the edges of the mind but without disturbing the focus, I guess I can do this a few seconds. But again I don't practice this, it was just from trying right now with no preparations.

>specific training/method did you use to attain your level of proficiency in both exercises (book, course, etc)? - Be as specific as possible
Qi Gong in Falun Dafa system for 15+ years, Ashtanga/Hatha yoga since 2019. Active bible study last 2 years (if you consider this a practice, I let this seamlessly fit into what I already do, and I achieved what I wanted with this for now).