How will voting help?

It's not really going to do anything, but it's better than legitimising the system and entrenching genociders. Politicians use the amount of votes they receive as an argument for the legitimacy of their policies. The overarching oligarchy will exist wether you vote or not, the individuals in the structure have a much easier time of doing heinous shit if they are perceived as legitimate.
So voting for the people in power will, in a small way, make things worse.

People are arguing voting does not work because if anyone actually leftist becomes popular, then they either get buried or subsumed by the structure - Bernie Sanders is an excellent example of both. In 2016 he got cheated out of the nomination, likewise in 2020. By 2020 he had already become a sheepdog for the left, channelling popular sentiment towards the establishment.
Another example is AOC who worked as a DC staffer for a long while, then took a 6 month gig as a bartender, and then received considerable promotion by the DNC by way of constantly being discussed negatively by the party elite, which then makes her appear as a political outsider - All the while she still supports the fundamental structure of the US as well as the DNC. She was a diehard supporter of Joe Biden as well as voting for Israel whenever it was needed, and running defence for things like the "Iron Dome".
However, others would argue that voting can function as a tool to build momentum for leftist political organisations.

Here's some Lenin for you. The text is very specific to the situation his party found itself in in that specific historical moment, but it can still be used to exemplify some stuff:

The principal question for Social-Democrats who value the elections primarily as a means for the political enlightenment of the people, is, of course, the ideological and political content of all the propaganda and agitation to be carried on in Connection with them. That is what is meant by an election platform. To every party at all worthy of the name a platform is something that has existed long before the elections; it is not something specially devised “for the elections”, but an inevitable result of the whole work of the party, of the way the work is organised, and of its whole trend in the given historical period. And the R.S.D.L.P., too, already has a platform; its platform already exists and has been naturally and inevitably determined by the Party’s principles and by the tactics which the Party has already adopted, has already applied, and is still applying, during the entire period in the political life of the nation which in a certain respect is always “summed up” by elections. The platform of the R.S.D.L.P. is the sum total of the work which revolutionary Marxism and the sections of the advanced workers who remained faithful to it have accomplished in the 1908–11 period, the period of the orgy of counter revolution, the period of the June Third, Stolypin regime.

The three main items that make up this total are: (1) the programme of the Party; (2) its tactics; (3) its appraisal of the dominant-ideological and political trends of the given period, or the most widespread of them, or those which are most harmful for democracy and socialism. Without a programme a party cannot be an integral political organism capable of pursuing its line whatever turn events may take. Without a tactical line based on an appraisal of the current political situation and providing explicit answers to the “vexed problems” of our times, we might have a circle of theoreticians, but not a functioning political entity. Without an appraisal of the “active”, current or “fashionable” ideological and political trends, the programme and tactics may degenerate into dead “clauses” which can by no stretch of the imagination be put into effect or applied to the thousands of detailed, particular, and highly specific questions of practical activity with the necessary understanding of essentials, with an understanding of “what is what”.
(...)
Consider the question from, so to speak, the general historical standpoint. It is obvious (to all, except Larin and a handful of liquidators) that the bourgeois revolution in Russia has not been consummated. Russia is heading for a revolutionary crisis. We must prove that revolution is necessary and preach that it is legitimate and “beneficial”. This being the case, we must conduct our propaganda for political liberty so as to pose the question in all its aspects, formulate the goal for a movement that is bent on victory and not one that stops half-way

Basically: Elections can function as a platform for a well-organized, principled leftist party to agitate to the masses and legitimise itself in the popular culture. So in the current day if you vote, you should vote for a leftist third party to help them build momentum.

Here's another snippet I think would be a good starting point for you, if you actually wish to understand the leftist position on elections, and not just argue against ridiculous strawmen. Really the whole book this second snippet is from is a good start. It's not too heavy either.

But very tl;dr:

• Elections are controlled by capitalists and cannot be used to leave capitalism.
• If you have a principled leftist party, then elections can be used to build popular support and momentum, but change will still be wrought from outside the electoral system
• The democratic party is not a principled leftist party
• Capitalist parties cannot be changed from the inside
• If you vote, vote third party to help build alternatives

For reading:

• Lenin "The Election Campaign and the Election Platform" and a snippet from "Leftwing Communism - An infantile Disorder](https://www.marxists.org/archive/lenin/works/1920/lwc/ch07.htm) and here's The book
• Luxembourg "Reform or Revolution"