Use the launcher's instance controls
Look for an option such as open folder or game directory on the selected profile. This is more reliable than guessing a global path because launchers and modpack managers can isolate each instance. Confirm you are opening the same profile that you intend to launch.
Understand the default path
On a default Windows installation the Minecraft directory is commonly under the user's AppData roaming folder, but custom instances can be elsewhere. macOS and Linux use different locations. The important folder is the active game's directory, with mods directly inside it.
Keep instances separate
Install the compatible Placebo jar only where it is required. Copying all jars between unrelated game profiles mixes Minecraft versions and loaders. For a dedicated server, inspect the server's directory separately; its mods folder is not the client's folder.