Just ask them to name specific imperialist wars they have waged and then debunk them.

Every war Russia has engaged in started as a civil war caused the dissolution of the USSR.
Ukraine was a civil war and russia only entered because the west refused to hold kiev to follow through with peace agreements that they had signed.
Georgia was similar. After the USSR fell apart the government of Georgia was let go but the minority ethnic groups in the north didn't want to be ruled by the others. Fighting ensured Russia eventually stepped in and the separatists joined Russia.
So even if the wars Russia has engaged in have expanded their borders it is not because they are expansionist it is because the borders were dumb and the brake up of the USSR was done in a slipshod manner.

Tibet willingly joined China, the only real fighting was terrorist uprisings by former slave owners, armed by the CIA, who were mad about the end of the theocratic slave state.
The situation in Xinjiang is the same operation only instead of Buddhist slavers, its a wahhabist al-qaeda splinter group, like isis, who want to enslave women and behead all "infidels" just like is going on in Syria right now.
Obviously Taiwan and Hong Kong are part of China (no involved party disputes it and if the person you are talking to does just lay into them for being ignorant fools who have no clue what they are talking about) If the government of China wants more political control over them that isn't expansionism.

I'm working on an introductory port for Russia here, specifically regarding Ukraine. It's far from finished but you can find plenty of useful sources in the comments.

As for China, I'd just ask them what "expansion" it's been doing for the past few decades. If they're referring to China's projects in Africa, helping countries move beyond resource extraction and basic refinement to higher levels of commodity production does not in any way align with the goals of imperialism.

One way is through immanent critique:

https://redsails.org/on-dialectics/

It may be worthwhile in some cases to attack the underlying subtext rather than debating point-by-point rebuttals. For example, starting with how China has the largest democracy in the world or how they have lifted 800 million people out of poverty - you will be challenging their understanding of democracy and what a fuller participation of democracy means while tackling Western chauvinism (consider also looking at the sources as well):

https://redsails.org/losurdo-on-china/

https://redsails.org/the-xinjiang-atrocity-propaganda-blitz/

https://redsails.org/another-view-of-tiananmen/

https://redsails.org/friendly-feudalism/

https://redsails.org/economic-war-on-asia/

https://thetricontinental.org/studies-1-socialist-construction/

May be consider how learning from China can help them better organise themselves:

What are the strategic consequences of decisively rejecting the tripartite social theory advanced by Orwell, and adopting Marx’s all-encompassing one instead? The basic call to action looks something like this:

Stop accusing the masses of being “brainwashed.” Stop treating them as cattle, stop attempting to rouse them into action by scolding them with exposure to “unpleasant truths.”

Accept instead that they have been avoiding those truths for a reason. You were able to break through the propaganda barrier, and so could they if they really wanted to. Many of these people see you as the fool, and in many cases not without reason.

Understanding people as intelligent beings, craft a political strategy that convincingly makes the case for why they and their lot are very likely to benefit from joining your political project. Not in some utopian infinite timescale, but soon.

If you cannot make this case, then forget about convincing the person in question. Focus instead on finding other people to whom such a case can be made. This will lead you directly to class analysis.

https://redsails.org/masses-elites-and-rebels/

If you can "crack" the disinformation on China then Russia will be a much easier sell.

Edited to add, youtube links on why China is not imperialist:

Carlos Martinez: https://youtube.com/watch?v=vgqSWeGHX2U

Vijay Prashad: https://youtube.com/watch?v=FXq1l2QCEc0

As for Russia specifically, the claim is incredibly silly: https://mronline.org/2019/01/02/is-russia-imperialist/

As for China, no because imperialism means wealth and labor extraction (not just a solid market position), and terms need to be unfavorable to one country for that to happen. China is a major trading partner for most countries in the world, but as its seeking allies, it does not want to alienate them, and so keeps terms usually in favor of the other country.

On the topic of Russia, check out the sources i refered to in this response: https://lemmygrad.ml/comment/7072432

I think this article has some good analysis - https://english.almayadeen.net/articles/analysis/why-russia-and-china-are-not-imperialist--a-marxist-leninist

regarding China, I recommend the articles & education pages that Qiao Collective has collated (and some of their larger ones are stuffed full of credible sources including ones from western media for those still skeptical). The 101 for socialism with chinese characteristics (under /education) is pretty good.

I'm sorry none of these are short, however.

However I also don't think short quips without sourcing are good for dispelling short generalizations like "expansionism==imperialist"(for that specifically, gotta point to Grandad Lenin's Imperialism The Highest Stage of Capitalism, to explain/define rather than to correlate by appearances*, right. And also that, the last expansion/border skirmish of China was almost 50 years ago, so China=expansionism==imperialist doesn't even work -- unless the people you're talking to are considering Taiwan not part of China, which Taiwan doesn't even agree - only recently has there been a TW separatism movement. Actually, thinking about it, western propaganda regarding China and painting it as "expansionist" is a good segue back to Qiao Collective. They have good resources particularly for the balkanization/separatism claims of parts of China including but not limited to HK, Taiwan, Xinjiang""East Turkmenistan"", Tibet, and also explainers for BRI also -- my tldr for BRI is that it's a multitool that does many things in regards to anti-imperialism, from eroding US dollar hegemony to assisting materially in development for underdeveloped places)

*I know it's sisyphean talking to western leftists and even newbie commies but. god. why is it so common for people's understanding of imperialism being, like, aesthetic. Imperialism is when bigger country does anything with/to another smaller country. By virtue of power imbalance, there wasn't real consent it means the smaller party was coerced. Expansionism means not just territory acquisition it also means spreading influence and making allies. It's frankly absurd.