If you want an extensive vocabulary in your target language, I highly suggest reading. Not just reading, but compelling content. The type of reading that you actually look forward to. It doesn't have to be the best thing ever, but engaging enough to where you start seeing pictures in your head.
There's a technique that people use when learning a new language and that's using pictures instead of words to build your vocabulary. This is why some people will use pictures when making flashcards instead of using words to give the meaning of the target language word.
The problem I have with flash cards is they're static. It's a still image of something. So an action like running can only show a still image of a person running. Still images can still be effective, but moving images are highly effective. Where's the best place to make moving images? In your head. Sometimes you will know what a word means because you have a picture in your head, but you will have to look the word up to give a definition. We sometimes do this in English. We know what something means, but we can't explain it or put it into words. We would actually have to find the meaning in a dictionary to put it into words.
If you're reading a really compelling book, you start to see a movie-like event play out in your head as you read. When you look words up, you may forget the meaning of the word, but if you see a movie play out to describe what the word means, you may remember it better because you see the thing happening in your head.
So let's say you are studying Russian and you're reading one of your favorite English language novels translated into Russian. You come across a word that means "to sit up". It's not 2–3 individual words, but one word. When you look up the definintion, you see that the word means "to sit up". In your head when you see that word, you picture the character in the story laying down and when the word comes up, you see the character sitting up in your head.
This is why I like reading novels. Novels use very colorful words. They can say one thing with 5 unique words. If you read a lot of novels in your target language, you'll have an extensive vocabulary on the level of a native speaker. When you read a lot of novels, you'll notice that you can understand a lot of words compared to people who rely on common word lists. The Harry Potter Book series (English) has around 12,000–15,000 unique words throughout the series. A native-speaking adult of English knows about 20,000–35,000 words accoring to A.I. Bing search. A voracious reader (someone who devours books) knows around 40,000–60,000 unique words. This may vary in your target language because some languages have more (or less) words than others.
So let's say over a 5-year span of learning your target language, you've read around 20–25 novels with around 500–1,000 pages per book. You would be able to understand more of the language than someone who focuses solely on speaking the language. You may even have native speakers in your target language kinda looking at you funny. Not because you said the word wrong, but because it's an unfamiliar word to them! We've been around well-spoken, well-read immigrants who have used words that we're not familiar with or we understand, but ever rarely use.
This is why it's very important for you to ONLY read books that you enjoy, so you will get to a point where you start seeing little movie scenes play out in your head of what you're reading.
As your vocabulary grows, you'll start to notice how much you can understand what people say even though they're speaking fast. Before learning the language, it all sounded like gibberish to you. After reading for a few years, you may not be able to follow what they're saying 100%, but at least it sounds like coherent words being spoken to you. You just need to develop your listening skills and know how to "listen fast" so you can keep up.