I have considerable experience in the housing finance industry, and I can tell you that unless provisions are made in advance (often with a fee to set up the escrow account needed), you cannot make partial payments on your mortgage. The borrower might think "well, it's not as though I'm not paying anything at all, I am willing to pay, I just can't pay the whole thing right now, I'll make another payment in two weeks". It doesn't work that way.
Absolutely, technically speaking, if you fail to make your payment by the first of the month (or whatever date is on your mortgage, it's usually the first), you are in default, and the entire amount of the mortgage is due and payable (acceleration clause), otherwise the lender can foreclose. As a practical matter, you have fifteen grace days, and then there is a late charge (usually 4 percent) for the period from the 16th to the end of the month. You have people who pay that late charge month after month --- that's how overextended they are. And again, as a practical matter, the lender will "work with you". They do not want to foreclose. Banks don't want to own houses. Banks don't want to have to sell houses to get their money. Nobody wins that way (except buyers at foreclosure sales who can get houses "on the cheap"). It is very rare for a foreclosure to take place within six to nine months. A year is more like it.
This industry is heavily regulated and audited out the wazoo, so I'm sure huge allowance is being made in the industry these days, due to CÖVÌD-19 and all the lost income that has come with it.