In 0.12.20:

1. In peaceful mode enemies never expand.
2. /c game.local_player.surface.set_multi_command{command={type=defines.command.build_base, destination={250, 100}, ignore_planner=1}, unit_count=10} Size of the base is selected by unit_count. The biters that are used for building are not spawned by this command, there needs to be a nearby spawner that provides the settlers.
3. /c game.local_player.surface.create_entity{name="biter-spawner", position={40, -40}}

Outside peaceful mode the biters select a candidate position, and send a group there once in a while. The interval depends on evolution factor and is between 60 minutes and 5 minutes (controlled in map-settings prototype in game data).

Positions selected needs to be at between 3 and 7 chunks from existing base and at least 3 chunks from player buildings (controlled in map-settings again). You can view chunks that are candidate for expansion wit debug view (F5 - F7 and F4) as green circles the map, the option name is called show_enemy_expansion_candidate_chunks.

Yes.

I know that pollution provokes Alien attacks, but overall, this is my character doing the pollution.

Not really. It is the things that your character builds that are doing the pollution.

Various structures in Factorio emit pollution (though only when running). You can actually see exactly how much pollution they emit by hovering your mouse over them and looking at the “Pollution” number in the information bar:

The wiki’s article on pollution has a good explanation for how generated pollution influences biter behavior:

Pollution attracts biters to your factory. Biters who find themselves in a polluted area will attempt to reach the source of pollution and destroy it.

Each spawner absorbs 20 + 0.01 * [chunks pollution] every game second (60 ticks), if the chunks pollution is greater than 20 otherwise it absorbs the total pollution.

Higher pollution values decrease the time it takes for biters to join the attack force. After a certain amount of pollution is absorbed the spawner sends one of its biters/spitters to a rendezvous point. Every 1 to 10 minutes (random) the mustered biters launch an attack. If not all biters have arrived at the rendezvous point by that time, they will wait up to an additional 2 minutes for stragglers.

When you open the ingame map (by default bound to the M key, you may need to press alt to enable the detailed view), pollution will be shown as a red-tinted cloud. The more pollution in an area, the darker the shade of red will be.

When the pollution cloud reaches biter bases, they will begin to amass to attempt to remove the source of pollution, whether you are at home or not. Build walls and turrets to keep your base safe, even when you’re not around.