So in my game, I'm a warlock who is a big fan of the darkness spell. But I want to use it in more creative ways. Does the darkness spell have to be a 15ft radius, or could u make it smaller? For example: if I wanted to create a darkness sphere around someone's head only or something like that

Must be a 15 ft radius. Spell says "fill a 15' radius", rather than 'upto'.

 The sphere of Darkness can be smaller if the space that it is cast in is smaller, otherwise what Mog_Dracov said is correct.

Quote from Brian_Avery >>

The Darkness spell is a magical spell that creates magical darkness.

Does this obey fantasy physics and stop at a wall if cast in a 10 foot by 10 foot room, simply filling the entire room and ending at the walls... or does it go beyond into other rooms and hallways for 5 more feet to finish it's area of effect?

Perhaps the wall stops it from extending further at the moment but if a hole was made in the wall or an entire wall knocked down then the spell would extend to it's full range?

This, like everything in the game, would require a DM's adjudication as I can see it working differently from game to game.

It behaves just as other spell effects. The base rule is that it can't effect areas that have total cover from the point of origin, so a wall would stop the effect spreading.

From the PHB:

A spell’s effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn’t included in the spell’s area. To block one of these imaginary lines, an obstruction must provide total cover, as explained in chapter 9.

However, the Darkness has the "spreads around corners" addition which means that if there is a door or a hole then it would spread as much as it could before hitting more blocked paths.

And the SAC goes on to specify that such spreading is still limited by the spells original radius, which in the case of darkness is 15ft.

Can a spell such as fireball go past its 20-foot radius if the point of origin is set in an enclosed space that’s less than 40 feet across?

The fire of the fireball spell can spread around corners, but it’s limited by the spell’s 20-foot radius. It doesn’t extend farther than 20 feet from the point of origin no matter where it is cast.

Edit:
As the spell has a duration I would allow it to spread (through a door or hole or similar) if the opportunity arose at some point during the duration but after casting.