How can the sixth channel of dhatus be understood? Could someone help clarifying the sixth channel of the eighteen dhatus - that is -  the mental objects (dharma-dhatu), the mental faculty (mano-dhatu)  and the mental consciousness (mano-vijñāna-dhatu)?

I’d be grateful for a more detailed description  of their nature and relationship, and if possible the relevant suttas that mention these.
OP: Could someone help clarifying the sixth channel of the eighteen dhatus - that is - the mental objects (dharma-dhatu), the mental faculty (mano-dhatu) and the mental consciousness (mano-vijñāna-dhatu)?

Here is a good explanation:

The element which is mano-dhatu or mind-element is nama. Mano-dhatu comprises the panca-dvaravajjana-citta (five-sense-door-adverting-consciousness) and the two types of sampaticchana-citta (receiving-consciousness) which are kusala vipaka and akusala vipaka. Thus three kinds of citta are mano-dhatu.
Dhamma-dhatu comprises both nama and rupa. Dhamma-dhatu is the cetasikas, the subtle rupas (sukhuma-rupas) and nibbana.
Mano-vinnana-dhatu or mind-consciousness-element is nama. It comprises all cittas which are not the dvi-panca-vinnanas and not mano-dhatu. For example, santirana-citta (investigating-consciousness), mano-dvaravajjana-citta (mind-door-adverting-consciousness) and the cittas performing the function of javana such as lobha-mula-citta are mano-vinnana-dhatu.
Panca-vinnana-dhatu (seeing-consciousness-element, etc.), the mano-dhatu (mind-element) and the mano-vinnana-dhatu (mind-consciousness-element) are: vinnana-dhatu (consciousness-element).
Dhamma-dhatu is not identical with dhammarammana (mind-objects). Cittas are included in dhammarammana, but not in dhamma-dhatu, since dhamma-dhatu is: cetasikas, subtle rupas and nibbana. When cittas are classified as elements they are the seven classes of vinnana-dhatu, namely:
Panca-vinnana-dhatu (which are five classes), mano-dhatu (mind-element), mano-vinnana-dhatu (mind-consciousness-element).
Concepts and conventional terms (pannatti) which are included in dhammarammana are not dhamma-dhatu, because they are not paramattha dhammas. Only paramattha dhammas are classified as elements.

Abhidhamma in Daily Life by Nina Van Gorkom, Chapter 18 - Elements

OP: if possible the relevant suttas that mention these

This is found in:

Vibhanga
Dhatukatha