Freeview is terrestrial which means that it is picked up via an aerial rather than a satellite dish.
There are more channels on Freesat than Freeview, but they are NOT the same channels.
On Freeview you receive channels such as “dave” and ther UKTV stations, among others. On Freesat you get the area brand of channels, some more news ones, and more shopping.
Freeview has HD but this is limited to where you live.
It should also be noted, for ease of use, that freeviews channels are numbered 1,2,3 etc, whereas Freesat is numbered 101,102,103.
TV’s are availlable to buy with Freesat built in these days but that is a small percentage. Most have Freeview but if you feel extravagant you can get televisons in with both Freeview and Freesat built-in.