Minecraft's HUD shows your hotbar, health bar, hunger bar, armor bar, and more. The armor bar only displays the total strength of your full set. Each piece has its own durability though, and one can break without warning, which can kill you in a fight. An armor HUD that displays durability for each piece is really helpful.
Good news: a mod adds this exact feature. Here's a quick guide to install uku's Armor HUD.
1) Install Fabric mod loader
You need Fabric first since it's the only loader the mod supports. Mod loaders are programs that let mods work in Minecraft. Fabric is among the most popular.
Go to their site, download the installer, run it, select the latest Minecraft version (1.21.10 right now), and install. It adds a modded version to your launcher.
2) Find and download the armor HUD mod
Other mods show armor on the HUD too, but this one's top. Visit Modrinth, check versions, and grab the one for 1.21.10.
It downloads as a .jar file. Drop it into Minecraft's mods folder on Windows: "C:\Users\aksha\AppData\Roaming\.minecraft\mods".
3) Open the official launcher and start the modded game version
With Fabric installed and the mod in place, launch the game, find the modded version (different logo, says "Fabric").
Start that version, load a world, and the new armor HUD appears next to your hotbar. Equip armor pieces, and it shows each one with its durability.