"This mod mostly adds a ton of mechs, vehicles and variants, including new mechs you haven't seen elsewhere that are all Lore compliant for 3025," reads the Nexus Mods description. Factions will use the mechs that make sense to them, but occasionally they'll use another faction's gear—but again, only if it makes sense in the lore.
It's essentially a collection of other mods that creator TheHaribo has tweaked. It's heavily based on another mech variant mod called JK_Variants, which adds 67 new mech variants plus new weapons. Battletech Extended 3025 sounds like a more directed version of that, with specific mechs introduced at fixed points in the story. For example, the starting mechs are less powerful, which will make the opening missions even more challenging, and you'll probably be using light mechs for longer in your campaign.
As well as using JK_Variants, Battletech Extended 3025 resizes different mechs, pulls in performance fixes and AI improvements. If you've never modded Battletech before, it might be a good place to start, and it's recommended that you start a new game to see everything the mod has to offer.
You can grab it here, where you'll also find installation instructions.