Whether you need a UK ETA for transit or a layover in the UK depends on how you transit.
Even if you are only connecting through a UK airport, you may need a UK ETA if you must pass through UK border control to collect baggage, change terminals, leave the airport, stay overnight, or for any other reason.
If you stay in the transit area and do not pass through UK border control, you generally do not need a UK ETA.
Therefore, if you plan to transit or have a layover at a UK airport, it is important to check your ticket and transit route in advance.
If you are preparing for a trip or transit through the UK, check whether a UK ETA is required before departure and apply in advance if needed.