常见的是This is his first job which he is devoted to，而在比较正式场合，介词to会提前，强调搭配关系，结构更严谨。

This is the first job that he’s devoted to or This is the first job to which he is devoted.

devote to sth. 是固定搭配。

这个to是介词。

定语从句里面 先行词+介词+which 的结构。
这里用to是后面的动词搭配决定的，devote to

介词提到前边了。举个例子the place which I earn a living in和the place in which I earn a living。过去英语好像有规定介词不得置于句末，新概念三还说过这个事，好像在第一课的语法部分来着。现在没这么多讲究了