How is skeptical thinking used as a tool within critical thinking — and is that the same as radical doubt?

Skepsis is a tool to optimise knowledge: continuously tap your thoughts and existing knowledge for weak spots and check their legitimations — not doubt everything for its own sake.

Critical thinking can be framed as skeptical thinking (skeptisches Denken), but skepsis here is instrumental, a means to better knowledge, not an end:

• continuously knock on all your thoughts and your existing knowledge to find possible weak spots, and
• carefully examine the justifications (Legitimierungen) that are supposed to hold them up.

This is productive doubt aimed at strengthening what survives. It is not the same as extreme skepticism — doubting everything indiscriminately — which counts as an epistemic vice. The skill is calibrated: doubt enough to find the cracks, not so much that nothing can ever count as known.