They used the Think(Pad|Centre) brand and expanded it to other things (like IdeaPad).

They basically bought IBM's good reputation name and style and used it for both IBM-alikes and budget machines.

Some of it was good machines, some of it was crap.

They actually bought entire ThinkPad R&D as well, and have been subcontracted manufacturer for some time before that.

So Thinkpads are still designed in ex-IBM Lenovo Japan