Give me some advice, tips/tricks, ratings on my bike that almost face plant my ass into concrete yesterday

So I bought an old Koga Miyata for 50 bucks of the marketplace, stripped it and put on wheels (coaster brake in the rear) and tires of my old fixie. Went for a test ride yesterday and it felt amazing and fast af untill my chain snapped at high speed and I almost launched myself of the bike bc my dumbass tried to break and somehow the crank kept rotating before the chain got stuck in my rear wheel lol.
Maybe someone can explain why my chain broke and what I can do to prevent it. Front gear runs 44t which came with the bike, rear 16t of the other bike, which I read could’ve been the cause bc of the difference (the front chain rings seem pretty similar to the ones on my old bike in size which had 47t or 48t). Or maybe I just did a lousy job linking the chain could be the most obvious reason.

Anyhow, I’m planning on putting my fixed gear back on once I get used to the bike and go brakeless, give it a proper paint job, change the pedals, maybe try a different handlebar but for now I just try not to die while getting used to how freaking fast en light this thing is lol.

So I’m open to any advice or tips/tricks on what to (not) do.