Search up in the web section "(theme of your collage) aesthetic pngs" or just "aesthetic pngs" and all sorts of cool pngs will show up and they definitely add an extra pop to collages! Some users have Free PNG packs on their page so look for those too

Adding filters overtop of your whole collage can make it look very put together. If you search up texture png or white transparent you can find some cool ones!

Or you can save the collage when you're done, add a filter from Instagram or other photo apps you have and then reupload it to PC