Does this mean they aren’t replaceable?

Depends on the laptop. Some can be replaced but you need to open the case (voiding warranty usually). Others are a bit more integrated and more trouble than they are worth.

OK, it’s a Dell laptop if you have any knowledge of these?

What do dell day? I’ve followed strip down and rebuild videos from their site before. Also check if they do spares for it. There is a support section that you can put the model into and it lists any factory spares they do.

A battery is a battery. They go in, they can come out. How difficult it is is another thing. Apples batteries are now surface mounted iirc, so soldered onto the motherboard. Saves quarter of a mm or some rubbish
. Obscene waste. A dell should be ok I reckon. Dell will do it for you for a cost im sure, but it might not be cheap. Otherwise hunt around online. Just need to source the right battery and bust through that warranty sticker!