Disclaimer: I'm not a trained singer or voice coach and may be talking outta my a-- (now there's a speech pattern).
I didn't understand the head/throat/chest thing either until reading somewhere to place your hand where you want the voice coming from, hum/sing/talk, and feel for vibration. It helped me to place one hand on my throat, and one on my sternum and hum scales to get the feel of where that "chest voice" is. Just don't try to force a deeper voice, at best it'll sound off and at worst it can strain the vocal chords. Practice and vocal exercises (try the "boom your voice" youtube video) can make some difference over time, and it's helpful even with HRT because no amount of hormone therapy will change speech patterns.
Speech patterns are a big part of how people gender voices, and there's more to it than head/throat/chest. Some cis women have deep voices and even speak from their chests, but still sound "girly" due to inflection, filler words/phrases, and various things such as elongating vowel sounds, use of vocal fry (bad for vocal chords too), soft vs. hard "s" sounds in words, etc. It can be helpful to listen to both feminine and masculine speakers and pay attention to differences, just like paying attention to body language, clothing, and hair cuts.
