Hey there! Welcome to the Half-Life community! It's always awesome to see new people join the series! Half-Life 1 ran on an engine known as "goldsrc" (gold source), which was a modification of the Quake 1 engine. With the release of Half-Life 2, Valve released "Half-Life: Source", a port of Half-Life 1 onto Half-Life 2's engine, "source". This port was a rushed, hastily made mess that was extremely buggy and just all around a poor port. Fans who were disgruntled with it's quality started work on a mod for Half-Life 2 in 2009 called "Black Mesa: Source" with the goal of re-making all of Half-Life 1 in Half-Life 2's engine. The mod progressed for several years before a build released in 2014. By 2016, the team, now called "crowbar collective" compiled cash to purchase a license to the Source engine, allowing them to port the mod as a standalone game on steam. The major hurdle with this is although they own the rights to use the engine, they don't own the rights to the Half-Life IP, meaning that they weren't allowed to use any assets from Half-Life 1 or Half-Life 2. They had to build the entire game from scratch. They did way more than that. Black Mesa wasn't just a remaster, it was a re-imagining of the game that started it all. There's hours of new dialogue, where silence once stood, we now have over 5 hours of original music (in my opinion, one of the best videogame OSTs right up there with Hotline Miami), The maps were expanded to show the full scale of a realistic depiction of Black Mesa. Generic offices now had purposes, you could tell that every bit of design had painstaking thought and detail put into it. The final chapter of the game had been completely re-written to be as long as all the chapters preceding it, making the game roughly 3 times the size of Half-Life 1. I'd totally recommend playing it after you complete Half-Life 1, it's an absolute ride.

P.S. To avoid any spoilers, don't look at the store page or any reviews. If you want to see what it looks like, I've enclosed a link to the tram intro to take a look at on stream :)
https://youtu.be/6aaQ0pRRK9g?t=12