First of all you'll be extremely happy if you'll be able to build a simple transistor if SHTF situation wipes out all chips.
Then there would be a problem for a simple calculator, needless to say anything about a computer.
Chemicals, rare metals, vacuum pumps and chambers, silicon (which is like glass), cutting equipment. There is not only rock or suitable sand finding is invovled, extracting, and refining to a pure suitable form is needed.
All electronics now use lead free soldering. The problem with it, it corrodes without lead (approx. 7 years for failure).
Electronics has a limited life-span. The last modern computer goes out, you'll have to forget modern chips for a long time. Placing and routing millions of components without a computer is hardly possible. You'll need first computer for routing purposes.
Pure transistor computers were improved to the limit at the end of their era, so technologies would be suitable from that time. I have no data about production requirements for foreign computers, only Lithuanian.
1968 year, transistor computer "Ruta 110", needed 410km of wires, 16000 transistors and diodes.
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link to www.technologijos.lt]
A whole industry is needed for that. With add on called Ruta 701 the same model became first OCR computer. It means that capabilities are suitable for routing.
Now all you need is routing and mask production for lithography of first post-apocalyptic chip.
Simple...