USA to Canada after 90 days without re-entering it

There is no physical exit control when you leave the US via the land border with Canada, so your concern is valid.

However there is physical entry control for Canada, and Canada provides details to the US for all border crossings - so at least in theory the US will be notified of your departure. Presuming you then leave Canada directly (without re-entering the US) your entry to the US will be deemed to have ended when you entered Canada.

However it is possible for this process to go wrong. For example, if you're a citizen of multiple countries and you enter Canada using a different passport to the one you used to enter the US then it's likely the records will not be matched correctly. Even with a single passport there's always the possibility that something will not be matched correctly and your exit from the US may not be recorded.

For this reason it's recommend to retain proof that you exited the US, and have this with you the next time you enter the US. The odds of needing this is extremely low, but it's worth having "just in case". Such proof might include the passport containing your entry/exit stamps to Canada, a copy of the boarding pass used to exit Canada, a copy of the hotel bill for your first night in Canada, a copy of a rental car agreement showing pickup in the US and drop-off in Canada, etc.

According to the US CBP (added emphasis):

How do I report my departure if I have an electronic I-94 and depart via land?

If you have a paper Form I-94 and depart by land, you can turn the form into the Canada Border Services Agency (CBSA) upon entry into Canada or to CBP at the port of entry prior to entering Mexico. If you received an electronic I-94 upon arrival, a departure will be recorded if you depart via land and re-enter the United States prior to the expiration date of your I-94. If you are not a resident of Canada or Mexico and you receive an electronic I-94 and depart via land but do not re-enter the United States prior to the expiration date stamped on your passport, you may want to travel with evidence of your departure into Canada or Mexico. Evidence of departure can include, but is not limited to, entry stamps in a passport, transportation tickets, pay stubs or other receipts. A traveler can request an entry stamp from the CBSA when entering Canada or from the Instituto Nacional de Migración (INM) when entering Mexico.

You can check your travel history on the same website, this page. If the exit ends up not being recorded you can try and contact them to correct the details. On the same FAQ page there's also details on what to do when the information recorded with the CBP (specifically, your exit) is incorrect - that's the last question.

Re to the second question - the US and Canada are separate countries with separate unrelated immigration laws. Your admission to the US for 90 days doesn't affect your admission or length of stay in Canada, it least by law. If the Canadian border officer is concerned about the overall length of your trip and your ability to provide for yourself - they'll inquire with you when you cross the border.

You may be confused by the fact that when you travel to the US on the VWP (with an ESTA), you are limited to 90 days per stay, and travel to neighbouring countries and islands (including Canada) does not “reset the clock”.

But this is specific to the situation where you go back to the US shortly after you left, and arrive from one of those countries. In that case, unless you can prove you actually travelled elsewhere, your new stay in the US will usually be considered a continuation of the original one, and will be limited to 90 days from your first entry.

But this is not your situation: you intend to go back to Europe, and you will not re-enter the US shortly (and if you do, it should be from a non-neighbouring country), so your stay in the US will end when you exit to Canada, and the only possible issue is making sure that this is correctly recorded (see the other answers for that).

In your case, the only complication which may arise is if your plans change and you need to re-enter the US shortly, arriving back from Canada. In that case, depending on the precise timing, you may be admitted for a very short time or even not at all.

Note of course that all this is based on you flying back directly from Canada to Europe. Any transit via the US would be considered an entry in the US (subject to the restrictions above), there is nearly never sterile airside transit in the US, and it’s even less likely from Canada (due to pre-clearance).