How is data divided into packets?

How is data divided into packets?

Hi sorry if this is a stupid question (I just started learning network
programming), but I've been looking all over google about how files/data
are divided into packets. I've read everywhere that somehow files are
broken up into packets have headers/footers applied as they go through the
OSI model and are sent through the wire where the recipient basically does
the reverse and removes the headers.
My question is how exactly are files/data broken up into packets and how
are they reassembled at the other end?
How does whatever doing the reassembling know when the last packet of the
data has arrived and etc?
Is it possible to reassemble packets captured from another machine? And if
so how?
(Also if it means anything I'm mostly interested in how this work for TCP
type packets)
Any pointers towards resources is much appreciated, thank you!