To some degree, kids need you to act interested when you aren't; you can't fully avoid their 'boring' talk without it leaving them feeling unheard or unappreciated. There have already been two great answers on how to participate in the topics you aren't interested in that I fully endorse and thus won't repeat.
That being said, your kids are old enough that you get some freedom to start trying to drive the conversation to things you're more interested in, after you have done your due diligence and listened - and ideally found a way to participate, even if you aren't that interested - to whatever obsession the child wanted to ramble about.
Let them know your ignorance to a topic
It's okay to say something like "Oh videogames, I don't really know about those" or "I always was bad with yo-yo as a kid" etc. This isn't saying 'no we can't talk about that', but it is your 'subtle' indicator to a child that this is not a topic you are going to be able to contribute in. By itself that won't do much, but it does lead into my next advice...
It's okay to say you want to talk about something else after listening to them for awhile
Shutting a kid down every time they want to talk about something you don't like isn't a good idea, the kid needs to feel you value them and thus their thoughts and interests. However, a kid also needs to learn that they can, and should, strive for conversation topics everyone enjoys.
So the compromise here is to listen for awhile, show interest in them and what matters to them. Then, after a long enough period that you have clearly demonstrated that interest and attention, you can then politely remind them that you don't understand video games and so can't really participate in the discussion. Now you can ask if they want to talk about some topic you know you both have a more shared interest in.
Yes, I am suggesting being a bit overt when you change the topic, you should be politely saying "I can't really be part of this conversation, so let's talk about this other one instead." Overt and blatant, if polite, is perfectly fine with a younger child such as your 9 year old. Even if being that overt with, say, an 18 year old would be mildly insulting, it's okay with a child who hasn't learned enough about conversations to pick up on the more subtle nuances of them yet. Basically, you are modeling how conversations should go, by giving a hint early on that 'this isn't a topic I can/want to be part of' and later reminding them of that when you change the topic so they can slowly pick up that they need to adjust their own topic of conversation based off what the other person in the conversation demonstrates an interest and desire to participate in.
Again though, you only get to do this after you did your parental duty of feigning interest in their topic first. I'll also point out it won't always work, quite often a young child will miss even an overt request to change the topic and if that happens, well you just sort of have to deal with it. Again at 9 they don't fully get that what interests them isn't interesting to you. You're modeling how the conversation should go, they won't always pick up on it. As the parent you're still stuck feigning interest if the kid insists on a topic, because making them feel heard is the most important thing you can do as a parent.
You get one veto topic
You can't ignore everything a child has an interest in, but presuming a child is interested in numerous things you can pick the one thing you are least interested in and keep reinforcing that this one topic is one you can't really do. So long as it's just one thing you stay consistent on and there are plenty of other topics the child is interested in that you do allow them to discuss with you, the child should be able to understand the distinction between 'really don't want to talk about this topic' and 'not interested in you or your passions.' Just don't make your 'veto' anything too generic, like all video games if your kid is an obsessed gamer.
I do have a sort of Minecraft veto with a nephew. He gets to discuss it a bit with me, but then I tend to redirect those conversations sooner than others with a reminder I really don't get/like Minecraft. That does mean that now that he has a Zelda obsession I'm stuck discussing that video game, and it's 20 different iterations, with him since I've basically 'used up' my veto with Minecraft, but at least I know Zelda and a lot of it's earlier iterations so I don't feel quite as out of my depth. Zelda may not be my ideal conversation topic, but it's better than Minecraft IMO so I consider it a win.
Of course as time goes on you can manage a more granular process of having an 'absolutely don't want this' response to one topic and a "okay we'll talk awhile but not as long" on this topic etc, you aren't actually limited to exactly one and only one veto. However at first stick to one, give the kid time to adjust to just that, and maybe later you can add a little more nuanced distinction or other things you want to avoid. The main reason I say you get one veto though is to stress this is not something to abuse. Such a veto should be limited to only the worst most offensive topics to you, you must still participate in discussing the majority of things the child wants, even if you find many of them boring.
Non sequential can be used when you really need a change of topic
Kids don't pick up on 'subtle' attempts to redirect them well, but they can still be easy to redirect. Rather than the politer sort of redirection you might use with an adult you can use the bludgeoning approach of completely derailing their conversation with something else. After you have participated in their passion for a bit wait until a slight lull in their rambling and throw in a "Oh, hey, I just remembered I wanted to ask you about whatever"
It's a good idea to have a specific discussion already planned out to redirect them, one you are pretty sure they would be interested in participating in and ideally with a good 'excuse' for your non sequential change of topic. I've been known to use the time a kid was talking to think about the change of topic I was going to use and how to most organically force it in.
I find topical things, like discussing plans for some upcoming event or letting them know an interesting fact about a place or time, work well since they flow more naturally as an "oh I just remembered this" non sequential. With the kids I regularly visit/mentor I also usually have one or two mini topics already picked out for the kids intended to teach them something that isn't being taught in schools so they generally expect a random "oh were suppose to talk about this on this visit" at some point during the day anyways.
So long as the topic is one that might be interesting to the kids this usually works. The catch is you can't overuse it, as I keep repeating you do need to participate in their hobbies and interests first before forcing another topic.