Most salvaged EV battery packs are dismantled for use in conversions, simply because they generally won't fit into a vehicle body which is not designed specifically to accommodate them... even the relatively small plug-in hybrid packs
The "battery packs inside" are called "modules". While breaking apart a module into cells is normally difficult, as long as they kept as intact modules the wiring is all plug-in connectors (for BMS) and terminals with bolts (for power), so they are practical to re-arrange.
When making a custom pack of these modules, you need to consider
- structural support and retention,
- electrical connections and supporting components such as fuses and contactors,
- enclosure for protection of the modules, protection of people, and containment of cell failures, and
- thermal management (such as heat transfer plates and circulating coolant).
