Greetings,
For any recent newbies who are interested, we can start with a very b r o a d overview.
(The best how-to information is in Bruce's 5th book, the guidebook.) Yes, your question
is very b r o a d !
Retrievals are a big purpose of being able to move freely between the physical and the afterlife. The act of retrieving 'stuck souls' (ghosts, spirits) is something anyone can learn,
it involves you with the afterlife's higher helpers and guides, and gives you a reason to visit and get to know the afterlife's various 'Focus Levels' where healing, reunions, and celebration occurs. Since Pure Unconditional Love is your main aid, your own soul grows immeasurably from these contacts.

If the process were simple enough to be fully explained in a paragraph, Bruce wouldn't have written a whole book (series), so please don't see this as a how-to course, but a retrieval involves notifying Helpers you are available to do a retrieval and firmly setting your intent to do one. (There are breathing techniques and PUL reminders to go through in preparation.) Soon you will find yourself out of the physical with the sense of a Guide at your shoulder flying/ushering you to the general area where a soul is 'stuck.'
Guides, helpers, angels cannot approach a stuck soul because the vibrational level of that soul has been damaged by trauma, fear, disbelief, etc. The role of a retriever is to become a link between the helper and the stuck soul, calming it enough that it can ascend or be approached by the Helpers who then get it into the afterlife.
You will find so much wonder, exchanges of PUL, and such understanding developing ! Your soul feels such release and acknowledgement to be freed of its physical casing and able to help with such necessary afterlife work !
Partnering, partnered explorations are essentially different. Our 'work' in afterlife is sort of at the 'pioneer' stage, right? Maybe not NDEs, which seem complete in themselves. But other explorations are hard to discuss. Bringing into consciousness all these various aspects of the afterlife often goes beyond our ability to communicate. So some folks set a time a place to go into non-physical realms together (OBE, phasing, whatever). Then when they get back they compare notes and try to see how their Interpretors might mean the same thing but use diffeerent symbols, forms, etc for what is essentially formless in a physical sense. In this way we learn 'how wide a net' has to be cast to pick up meanings in non-physical reality.
If you would read Bruce's books, all this would be presented in his calm, experienced, organized, loving manner. It's much harder to get the import of this through bits and pieces.
Love, Bets