Also, how did this alter with the progression of the conflict?
Its a bit of a difficult one to answer because the size and shape of the platoon and its basic equipment changed an awful lot during the war, plus there were variations with between British and the various Commonwealth contingents, and even variations in how platoons would ad hoc their equipment.
In 1914, the general pattern was for a platoon to have 5 sections - 4 of these would be rifle sections, each with 10 men and headed up by a corporal, and then an HQ section with 4 men and a subaltern officer.
Men would be equipped with a Lee-Enfield rifle and bayonet, and the officer would carry a revolver (and sometimes a sword). The officers would tend to dress and arm themselves in an identical manner to their men as the war went on, to avoid making targets of themselves to enemy snipers.
Further developments:
By July 1915, the battalion heavy machine guns (Vickers) were being centralised and replaced by Lewis guns - initially a straight swap at a rate of 4 per battalion but by June 1916 this had risen to 16 per battalion. At this stage they were still Company assets, but by early 1917 with the publication of the doctrinal pamphlet SS143 "Instructions for the Training of Platoons for Offensive Action", Lewis guns were being issued at the rate of 1 per platoon and these were now seen as platoon assets. This would increase to 2 per platoon, though it was not unheard of for platoons to "borrow" some extra Lewis guns where possible.
These guns were operated by specialist sections within the platoon, though Ivor Maxse was advocating from an early date for generalist, rather than specialist sections with 7 to 8, instead of 10 man sections.
So what were the other specialist sections doing? Well, a lot of the fighting in WWI was a matter of bombing, or grenade fighting, rather than shooting. So one of the sections would be specialised bombers.
Grenades became A Thing in the British army early in 1915, though the first grenades were primitive "jam jar" type grenades. They were made literally from old jam jars filled with explosive, old nails, and with a fuse that had to be lit by a match or cigarette sticking out of the top. After much experimentation, the Mills Bomb was developed, which is essentially the hand grenade as we know it today.
As rifle grenades became a thing, another section would be specialised in using these as fire support - almost like mortars - though they could also be used to fire flares to signal to spotter aircraft or artillery.
The standard bayonet was a fearsome weapon, but not really suited to the close confines of a trench. Hence soldiers would carry a variety of hand to hand weaponry, ranging from truncated spade handles with a cog and/or spikes on the heavy end, to custom manufactured "trench knives" - brass knuckles with a blade attached.
By the time of the 100 days offensive, platoons had evolved into what we see today, with approximately 8 men per section, and each section having a mix of generalists and specialists - bombers, gunners etc distributed evenly throughout. This gave the platoon the necessary flexibility to contend with the demands of modern warfare and the shift back to mobile warfare.