Bearings are often held in place with light friction fits. Some bearings have flanges to hold them in a hole. You can also cover the sides of a bearing with something to hold it down, like a screw with a washer, the edge of the washer holding the edge of the bearing down. For the inner race, you can keep it from sliding along a shaft with a shaft collar, held by setscrews.
Ball bearing races are hardened and cannot easily be machined.
You never want to have a situation where the shaft is rotating but the bearing isn't, or the bearing is rotating in the bigger housing hole. Thus the light friction fit. The idea is that a little friction will hold the bearing in place, and the very low friction of the bearing itself won't put much load on the frictional parts.
If you are using angular bearings, you are expecting some side load on the bearing. You then seen something more than a screw with a washer or a friction fit to hold it in place. If the load is in one direction only, a flanged bearing is great; otherwise you machine a hole that has a lip at the bottom, press the bearing into it, and use a cover (with a hole for the shaft!) to hold it in.
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