Note that not all products are Zigbee Router devices, however, as a general rule most mains-powered products are Zigbee Router devices (but not all), while all battery-powered devices will always be end devices (e.i. non-routers).
As mentioned, suggest start by reading and try following all the tips collected here as that should take care of most things → Zigbee networks: how to guide for avoiding interference + optimizing using Zigbee Router devices (repeaters/extenders) to get best possible range and coverage
It is primary a feature because there are some devices with buggy firmware that will not automatically move to a closer Zigbee Router device, but it can also be used because it can make it easier to add/join/pair a device that is far away.
By design all Zigbee devices should constantly evaluate its neighbours and and automaticlly migrate to a better Zigbee Router, and most devices will do so about once every 24-hours if are better choice for it.
So you can not force a device to keep staying with a specific Zigbee Router unless it has a badly written firmware with a such bug that prevents it from migrating.
You should not really need to bother with how the Zigbee network meshes, it should all be done automagically, and the only thing you should need to consider is adding enough Zigbee Router devices.
Normally you have no control what so ever over how a Zigbee network meshes, as the only thing you can and should do is to add more Zigbee Router devices.