With the XRay Fabric mod, you can look through the blocks and quickly find all the valuable resources. This will greatly simplify survival, but this feature is cheating and is…
XRay mod for Minecraft 1.15.2
Minecraft 1.15.2 (January 2020) is the last release before the Nether Update. An XRay mod differs from the texture pack in one thing: it has a switch. The world looks normal until you press X; then stone, dirt and the other filler blocks go invisible, ore and chests stay, and a second press brings everything back. The list of highlighted blocks is editable, so you can leave only diamonds and ancient debris, or add spawners and chests.
On 1.15.2 both loaders are covered. Forge players take XRay Mod for Forge, Fabric and Quilt players take XRay Mod for Fabric. The two share the same hotkey and the same block list, so switching loaders does not mean relearning anything.
Ore on 1.15.2 sits in the classic layers: diamonds under Y 16, redstone and lapis in the twenties, gold under Y 32. Stand at Y 12, press X, and the diamonds within render distance are the blue dots along the bottom.

Servers
Keep it off on public servers. Anti-xray plugins on 1.15.2 send fake ore to the client, so the mod shows garbage, and the mining pattern itself gets you flagged. Singleplayer, LAN and your own private server are the places for it.
Prefer no mod loader at all? XRay texture packs for 1.15.2 do the same job through resource packs.