As for the company galleries, the keyword that you use matters. It has to be one of the keywords that are defined for that company.
For example:
Let's say I made a card using Paper Smooches stamps. I could put "papersmooches", "Paper_smooches", "PS", "Paper Smooches", etc in the keywords. Take a look at
this page and find Paper Smooches in the list. Hover over the company name. You will see the two keywords that will put your card in the Paper Smooches company gallery. Using "Paper Smooches" or "PS" as a keyword will not.
However, there is a way around that issue...If you use "Paper Smooches" in the
search box on the gallery pages, then the cards that use that phrase anywhere in the post show up.
I would imagine that your cards that do not show up in the company gallery when you post probably don't have the company name in one of the "acceptable" forms.
As for where your uploads appear, all of them will appear in your own gallery. Beyond that, my understanding is that you may choose only one place to designate where that card appears. So, it can go in a stamp set gallery OR a technique gallery OR a project gallery, etc.
Be aware that putting keywords into your description is really putting in a search term, so to say. You can (and it's a great idea) put as many keywords as you want.
For example, let's say you made a card for a challenge on SCS, and it was also for several other challenges, and it used a certain company's stamps, and it was CAS (clean and simple) in its design, and it was for a guy, and it is a birthday card...you get the picture. ALL of those keywords could be used and anyone searching for those cards will see yours.