How do you use custom buttons and links in Lightning Experience? What replaced JavaScript buttons?

Custom buttons and links are configured in Setup > Object Manager > Buttons, Links, and Actions for each object.
URL buttons: Navigate to a URL, optionally using formula-based dynamic URLs that reference record fields. Example: a button that navigates to an external system with the record ID appended in the query string. URL buttons still work in Lightning Experience.
JavaScript buttons (deprecated): Used to execute JavaScript in Classic. JavaScript buttons do not function in Lightning Experience and must be migrated.
Migration options:
— Screen Flow: Replace simple JS button logic (pre-populate fields, navigate) with a Screen Flow invoked via a quick action.
— LWC Action: Build a Lightning Web Component invoked as an action for more complex logic requiring component UI.
— Headless Quick Action (LWC): An LWC component without a screen that executes Apex logic when the action button is clicked.
Use the Salesforce Lightning Migration Assistant or review the JSforce Locker Service documentation to identify JavaScript buttons requiring migration.