Assumable Mortgage Scenario (Explain like I'm 5)

[Kendric Buford]

I've been trying to wrap my head around assumable mortgages, but I'm not sure I get it fully. Can someone tell me if this scenario is accurate, or explain it like I'm a child?
Scenario: (using round numbers so I can follow better)
Seller is trying to sell their house for $300,000 but can't. Their Original Purchase Price was $250,000 at 3%. They currently have $50,000 in equity in the property. I offer to buy it by assuming their mortgage. Does that mean I'd take over the remaining loan of $200,000 at 3%, then I'd have to pay the seller $50,000 for their current equity, and an additional $50,000 to cover the difference from the initial amount they wanted to sell for ($300,000)? - Also, I presume this is where my cash and/or a second mortgage would come in, so I can pay the seller.

**To keep things as simple as possible, I didn't mention any closing costs or commission to an agent for this scenario.  

So at the end of all this, if I'm understanding correctly, the Seller would walk away with the $100,000 that they would have from a normal sale, but I get a 3% mortgage rate and the property?

[Randy Rodenhouse]

@Kendric Buford @Kendric Buford. I assume you’re talking about buying a house subject to the existing mortgage since assumable mortgages for the most part are not available. There are assumable mortgages, but you have to qualify for them (Meaning you have to go through the process of working with the bank, showing your tax returns, etc.)

If you’re buying the house subject to the existing mortgage and they have equity above that amount then u would buy sub2 and have seller carry back a note for the difference. So in your example, I would buy the house sub2 the 200k and get the seller to carry back a note for 100k if buying for 300k. However, buying at 300k would not be a good deal so instead I would negotiate to buy at 250k and buy sub2 200k and have seller carry back note for 50k. There are many variations. So if they wanted 10 k down then buy sub2 at 200k and have owner finance 40k and you give them 10k cash.