If I have a previous visa refusal from Country A, will it impact on my application to Country B too?

By lying to immigration, you have made your travel life much more complicated and much riskier. The reasons are two: first, it's difficult to keep your stories straight if you say one thing to one person while saying another thing to other people (and this gets progressively harder the more people and agencies are involved), and second, some immigration agencies share information. Consequently, the only advice you'll hear here is "Tell the truth."

If you'd been honest throughout, then having obtained a US visa in 2018 would bolster your current Canadian application, as the grant of a visa shows that the US authorities found your application trustworthy.

You said you didn't lie on the Canada application, so you will have disclosed in that application the rejection from Germany. But you did lie in the US application for the 2018 visa, and denied receiving the German rejection. Thus, the content of your US visa application is different from the content of your current Canada application.

The US and Canada share a great deal of intelligence information under the Five Eyes agreement. No one knows specifically what information, but the two countries clearly have the authority to share immigration information (see the discussion here), and anecdotal evidence from the US/Canada border suggests that immigration information is indeed part of the shared data stream. If information is shared (a perfectly reasonable possibility), and if the information is cross-checked as a matter of course (another perfectly reasonable possibility), then the Canadian visa authorities will see that you answered the visa rejection question differently on the US application than you did on the current Canadian application.

It's even possible that information about the German visa rejection is directly shared with Canada or the US, which would make the discovery even more certain.

Saying different things at different times to different recipients will be seen as misrepresentation; more brutally: they'll conclude you must have lied at least once. Discovering your misrepresentation is likely to result in the denial of your application to Canada. It will also do no good whatsoever for any future application for a visa to enter the US.

I don't know what might be done here, if anything. At this point, I'd stop asking questions of random strangers on the internet, and find an immigration attorney in Canada. You don't have to be physically in Canada to hire counsel there, and right now you want someone who knows the Canadian immigration system. If you want to apply to the US in the future, I think you'll then need a US immigration attorney.