What is described in the poem of sonnet composed upon Westminster bridge?

There isn't very much personification of London in Wordsworth's Composed upon Westminster Bridge, September 3rd 1802:- certainly not as much as one might expect in a typical poem of the period. In a sense, the lack of personification is an important point the poem is making.

There are a few light touches of casting London as a person: the city wears 'the beauty of the morning' 'like a garment', and near the end of the poem Wordsworth remarks 'Dear God, the very houses seem asleep / And all that mighty heart is lying still':- giving the city a 'heart' which clearly is not literally the case. But to talk of the 'heart' of a city is almost a dead metaphor (as is mentioning early morning houses being 'asleep').

Most of the language of the poem is very matter-of-fact, much closer to everyday speech than we would normally expect of a poem: "Ships, towers, domes, theatres and temples lie / Open unto the fields and to the sky". A major point that Wordsworth is making in this poem is that ordinary things can be more 'poetic' than fancy stuff can. A busy working port can be just as moving as a mountain sunrise, a modern city can touch you as deeply as any view in the countryside.

The poets who were in fashion while Wordsworth was growing up (Alexander Pope, Oliver Goldsmith, James Shenstone, William Collins, Samuel Johnson) had favoured a very flashy and ornate way of writing poetry:- full of clever turns of phrase, abstract nouns (often with Capital Letters), figures of speech, and personifications.

Wordsworth was deliberately writing in a much plainer language:- not quite the sort of thing you might hear in everyday speech, but very nearly so.

So there is not much personification in this poem, and what there is, is unimportant.