When I started? Mostly bugging my Japanese buddies all too much or just ignoring them. But that isn’t really very helpful.
After I got my head on a bit more straight with the reading thing, I broke terminology into three different parts. First was just standard vocabulary and grammar. For that, I used a English - Japanese Dictionary. I have a couple physical ones along with using jisho.org a lot. I had similar issues with limitations of what was in said dictionaries until I ultimately started using a Japanese - Japanese dictionary. That solved a lot of the issues I had with not being able to find words. For that, https://kotobank.jp/ and http://nlt.tsukuba.lagoinst.info/search/ were super helpful when I started. Apple’s native dictionary is also really good. Also, just googling 「言葉意味」 would sometimes work too. If that didn’t work, but if I knew a synonym of the word, I would just google something along the lines of 「A言葉とB言葉は何が違う」and that would often bring up what I was looking for. If none of that worked, then I would bug my Japanese friends, and they were always supper helpful.
The second thing that often came up was slang and accent stuff. For that, the first thing I would try to do is identify the speech level/ type of accent. If I could figure out that out, then I could just google something like. 「何弁、言葉、意味」and that would often find it. If that didn’t work, I would either bug my Japanese friends or other Japanese learners. To be perfectly honest, senior Japanese language learners where probably more helpful with things like that, especially if they studied in that area in the past.
Finally are names and made up words. I would try to look these up if I could, but if not I would just self define them in my head and try to brute force my way through them. I figured this one out after talking to an acquaintance who was trying to read Harry Potter in English and was having a hard time with all the made up words in that book.
That’s just a quick once over on everything, let me know if there is something you want me to expand on or go over. I’m not really that great at most of Japanese, but I do a lot of reading and am always happy to help.