What is forcing after opening a minor and partner bids a major, and I jump to 2NT?
This is very system dependent. See this article for example for a long discussion of various options.
Standard, everything other than pass or a game jump is forcing. Remember you have 19 and partner has 5-6 at least usually, so you’re probably in a game.  I would at least play Gerber here (so 4C is not natural) but 3 level bids are forcing and usually natural, other than 3NT of course which is to play.
The most common conventions I see here are to play various puppet options - basically every bid by responder transfers to the next higher suit at the 3 level (3c-3D-3h transfer to 3D-3h-3s), and either that is the weakest option (responder passes the transfer) or the stronger (if responder bids on).  Jump to game is middle. But it really depends on your tolerance for various systems -  it can get very complex, as is appropriate for a circumstance where you could have anywhere from a part score to slam, and one partner knows which.
The question is why you’d want 3D to be nonforcing.  Responder is totally 100% in charge in the auction at that point, so it’s not a question of forcing or not - it’s responder either tells you to bid again or tells you to pass.  You could make 3 level bids sign off, but that takes up a lot of space; you’d rather not if you can avoid it.  That’s why you use puppet bids there - it lets responder double dip and have both signoff and exploration bids both available for the same bidding space.