Vector scales, raster does not
A vector file (SVG, AI, PDF) stores shapes as maths, so it stays sharp at any size, a business card or a building sign. A raster file (PNG, JPG) stores pixels, so enlarging it produces the soft, blocky look you have seen on badly made banners.
Your logo should always exist as a vector. Everything else can be exported from it later; the reverse is not true.
CMYK for ink, RGB for screens
Screens emit light and mix red, green and blue. Printers lay down cyan, magenta, yellow and black ink. The two ranges do not match, which is why a vivid screen colour can print muddy.
If something is going to a printer, ask for a CMYK file. If it is going online, RGB is correct. Sending the wrong one is the single most common reason a print job comes back looking wrong.
The three things printers ask for
Bleed: extra artwork past the trim line, so a slightly off cut does not leave a white edge. Usually 3mm.
Outlined text: fonts converted to shapes, so the printer does not need your typeface installed.
Resolution: 300dpi for print, 72dpi is fine for screens. A web image blown up to poster size will not survive.