budget matters, how tech savy are you matters, as 5th ave mentioned, how wireless is wireless to you. There are battery operated camera's, but it gets expensive even with rechargeable, as they don't last that long.
There are also people that wire up battery packs to power non-battery operated camera's in hard to get at area's of the home or away from power. using inverters and POE type camera's.

also some expectations should be set depending on budget and what you plan on doing with the camera's as well.

night vs day, distance, PVR record time, clarity, notifications or remote viewing. All of these change the price point, some a little, some dramatically.

Piecing together a system can be done, but cost will be a factor. While it is true you are allowing flexibility of cameras, the capture portion depending on the camera can be expensive. There are "free" softwares out there that let you use generic IP camera's as well. but then they limit the ability to use them till you pay for that feature.

You should also be careful about "cloud" based storage or company storage like Ring.
If the internet goes out you don't get a recording of what happened.
like the old days where they cut your phone line and no alarm goes out, cutting your cable / internet means ring doesn't record what happened when they come to your front door.
a few others are like that wyze is like that, but they do offer the ability to put in a memory card in their camera's so at least you have local recording.

Reolink is the one I currently use, but that was more budget minded.
I have added on to the original system a few times, but may have to step up due to clarity issues, as well as speed to view on their apps.

all that was to say, what are you trying to use it for, and what kind of budget do you have in mind. how easy is it to run power vs actual cable connections?
As I have places I can run power easy, but to get ethernet back to a central place would be impossible.