Why big cubes use random moves
For the 3×3 and 4×4 a generator can pick a random position and work out how to reach it. For the 5×5 there is no practical way to do that, so the scramble is a long sequence of random turns instead: 60 moves, enough to mix every piece thoroughly. That is also how the WCA scrambles the 5×5.
Fixed centres make the orientation easy
Unlike the 4×4, the 5×5 has a fixed centre piece in the middle of each face. Hold the cube with the white centre on top and the green centre facing you before you start, and that orientation never becomes ambiguous.
Reading a 5×5 scramble
A w turns the outer face together with the layer next to it, and on the 5×5 any face can be turned wide: Rw, Lw, Uw, Dw, Fw and Bw all appear. With 60 moves, a single slip changes everything, so go slowly and compare the result with the preview.
Practising the 5×5
A 5×5 solve is long enough that consistency beats speed: an average of five steady solves is worth more than one fast one. The timer keeps a separate 5×5 session, so your big-cube numbers never mix with your 3×3 averages.