Anyone who steals bikes regularly knows trackers exist, and the obvious spots get checked first. The goal is a place that is out of sight, awkward to reach quickly, and still good enough for a Bluetooth signal to escape.
A holder mounted on the bottle cage bolts sits in shadow behind the cage and looks like part of the bike. It is easy to install, reversible, and the usual first choice.
A saddle-rail holder is hidden by the saddle itself and by your body while riding. Good for frames where the cage bosses are already in use. On saddles with a SWAT-style interface a dedicated mount fits even more tightly.
For e-bikes, cargo bikes and frames without spare bolts, a self-adhesive holder can go on a flat, clean surface: under the down tube, behind a battery cover, or inside a fairing. Clean the surface with alcohol and let the adhesive cure before riding.
A tracker is a recovery aid. It can give police a location, and it can tell you your bike left the building. It does not prevent theft, positions can be delayed or missing where few phones pass, and you should never confront a thief yourself — pass the information to the police.