The game allows both. You may play with friends, strangers or alone, all up to you.

This depends on the game mode. Please refer to each for more specifics, but in general:
Arena - PvP (and vs AI bots)
Prospector - PvE
Strategy - PvE & PvP

 Both FPS and RTS perspectives have been approached with the conviction that neither must be compromised - both must be true to their genres.
Rather than add a small FPS element to an RTS game or vice-versa, in Silica, it has been the plan to approach each as its own separate game to the extent that while in each perspective, you feel you are playing a game designed for such.
For example, playing in FPS perspective, you have weapon scopes, can crouch, sprint, jump, and hop in and drive vehicles just as you would expect in a standard shooter. Likewise, in the RTS perspective, you construct buildings, produce units, harvest resources, research higher technological tiers, and look at the scene from above, just as is expected of an RTS.
For fog of war, please refer to its segment.

Yes, the game has a fog of war that is reminiscent of classical RTS games and is influenced by terrain (hills, dunes, etc). It has 3 levels of concealment, listed below:
- Unexplored (black, nothing can be seen in it)
- Explored (partially transparent, can see discovered buildings but not units)
- Visible (transparent, can see units, buildings and so on)

Important to note is that the fog of war is not present in the FPS perspective, where everything is as one would expect of a standard FPS. The crossover is in how units are detected, where both perspectives share common data. A target (unit/building) is detected when either within view distance (the distance at which fog of war is revealed) or by line of sight (distance depends on object size, movement etc). The exception to this rule is units that are considered 'concealed', explained in the next question.

A unit is considered concealed if it is near rocks, cliffs and other large surfaces that would provide cover. This is dependent on the size of the unit and not all units may be concealed (such as the colossal Harvester). In general this applies to smaller units, such as infantry or basic aliens.
When a unit is concealed, it may be detected only via line of sight. This means that from an RTS perspective, even if the unit is within view range, it must be visible to your units to be detected.

Disregarding concealment, mentioned in a previous question, a building is detected when it is within line of sight or within view distance where fog of war is revealed. Units are also detected in the same way but their detection depends on how fast they are moving, how fast the observer is moving, which direction the observer is looking and so on.
 

Currently, servers are hosted by players when they tick the 'allow others to join' tickbox when starting a game. They play while hosting (aka Listen server). Dedicated server support will be added during Early Access. At present there are no official servers, though this may be considered in future.
Important to note is that NAT traversal is handled by the networking system, so no port forwarding or other setup is needed.
 

Depends on the game mode but in general:
Arena - Any human/alien unit against any human/alien unit
Prospector - Humans against humans or aliens
Strategy - Either of humans vs humans, humans vs aliens, humans vs humans vs aliens
 

Currently the following factions are present in the game:
Sol - Humans from the planet Earth in the Solar System. One of two settlements in the galaxy.
Centauri - Humans from the planet Centaurus in the Proxima Centauri system. The other of two settlements in the galaxy.
Aliens - The native inhabitants of Baltarus. Extremely territorial, they will aggressively defend their Queen's territorial boundaries.
 

Humans (Sol/Centauri) mine Balterium while Aliens harvest Biotics, both explained below:
Balterium - Located underground in clusters, it induces the crystallization of sand surrounding it, resulting in tall crystals that protrude from the surface, a tell-tale sign of its presence. Mined for its use as an ultra-dense energy source as well as its additional properties.
Biotics - Any biological matter, such as that from flesh, plants and tar pits.
 

Currently the following game modes are available:
Arena - Select your favourite unit and face off in this unbalanced, FPS free-for-all which doubles as a place where you can try out the various units in the game.
Prospector - Serving as an introduction to Silica, explore the dusty wastes alone or with friends and locate Balterium, encountering various objective on the way in this coop mode.
Strategy - The flagship game mode of Silica, battle for dominance by building, harvesting and attacking the enemy. You may choose to play as the Commander, controlling the battlefield akin to a classical RTS, or instead may decide to take on the role of infantry, utilizing the produced vehicles to seek out and eliminate the enemy. This mode is playable in any combination up to a 3-way conflict between the factions.