Who draws the steel beams for an extension?

Two different people are involved, and it matters which you need.

A structural engineer calculates what size beam you need - the loads, the span, the section size, the padstones and bearings. This is a calculation they are qualified and insured to sign off, and Building Control will want it.

A draughtsman / steel detailer (that's me) takes those calculations and produces the drawings your builder and steel fabricator work from - setting-out, fabrication details and connections. I do not size beams.

So the order is: engineer's calcs first, then detailing. If you already have your engineer's calculations, I can turn them into fabrication and Building-Control drawings quickly. If you don't, get the calcs done first - I can point you to an engineer.

Steel drawings for extensions (from your calcs)