An excellent primary source on this subject is available online: Field Entrenchments: Spadework for Riflemen by John Solano (first published in 1914, with updates throughout the war) can be viewed in its entirety thanks to archive.org. Have a look in particular at Chapter III (page 39) Entrenchment in the Presence of the Enemy as a starting point. This gives a very detailed description of how a full trench line can be constructed and developed, starting off with shallow, hastily grubbed-out shelter pits dug by individual riflemen while under fire. The Manual of Field Engineering, reprinted 1914 is another good source if you can find a copy; a few pages are available here.

A trench system would be incrementally developed and improved, day after day, with back-breaking labour, mostly by the infantry in the line, but with support from the engineers, Entrenching Battalions and other supporting formations like the Labour Corps. It's really this absolute mass of labour available from millions of men under arms that explains how these vast earthworks were created so quickly, with the driving factor of the desperate need for shelter from artillery. As for the complexity of the trench systems, there are two major reasons. Firstly, the requirement to have not only a strong defensive front line, but also to be able to supply and reinforce it and provide 'defence in depth', with the ability to counter-attack this line should the enemy seize a section of it; for this reason, an extensive network of 'communications' trenches was needed to move large numbers of troops, rations and ammunition under cover, and a series of support lines, reserve lines and redoubts were needed to provide further lines of resistance behind the forward trenches. The second reason is the sheer amount of supporting earthworks beyond what we think of as the main trench lines: you had the 'supervision' trench immediately behind the front, saps and tunnels, cover trenches alongside communications trenches in case of heavy bombardment, assembly trenches to form up troops for major attacks, dummy trenches and more. When an area was fought over repeatedly, all of these earthworks could end up incorporated into a local trench system, making it an incredibly complex maze:

When moving about in the trenches you turn a corner every few yards, which makes it seem like walking in a maze. It is impossible to keep your sense of direction and infinitely tiring to proceed at all. When the trenches have been fought over the confusion becomes all the greater. Instead of neat, parallel trench lines, you make the best use of existing trenches which might run in any direction other than the one you would prefer, until an old battlefield, like that of the Somme, became a labyrinth of trenches without any plan.

Subaltern’s War by Charles Edmunds (Carrington) (London, 1929) pp217-218

In a 'loud' sector where enemy observation and fire was too intense to achieve much by day, working parties would move up from the support lines under cover of darkness and work through the night. Reliefs (one unit taking over responsibility for a front-line trench from another) and movement of supplies would also take place chiefly at night. Any movement of troops or wagons was liable to draw artillery fire, and choke-points such as communications trenches and crossroads just behind the lines were generally known and pre-registered by the enemy artillery, even if they were out of direct sight, and would come under random bombardment in the hope of catching a party on the move and disrupting their efforts. Working parties in the line (or in front of the line working on the wire) that drew attention to themselves because they made too much noise or were spotted when flares and illumination shells went up would be shelled and machine-gunned. Ultimately, the casualties that all of this inflicted simply had to be borne and the work carried on, as without improvement and supply a trench line was vulnerable to attack.

Trenches and strong points might also be taken from the enemy, rather than dug from scratch, and reversed, joined up and incorporated into the existing front line. This again would be achieved with enormous loss of life and hard, frantic, terrifying, physical work. I mentioned a bit about how trenches were raided, taken and reversed in an answer to this question:

What happened after a successful attack against an enemy trench in WWI?

It wasn't all like this, however. There were plenty of 'quiet' sectors where working parties from both sides would allow each other to go about their business largely unmolested, in a spirit of 'live and let live'. Rations and other supplies could be moved in daytime, and in some cases weapons fire was even ritualised so you would know exactly when to expect a handful of enemy shells to arrive, fired to satisfy the reports back to HQ rather than to inflict casualties. Trench Warfare 1914-1918: The Live and Let Live System by Tony Ashworth is an amazing study of this facet of the war and well worth a read.