Forgive me if you appreciate this already. The battery is tested most when trying to start a cold car. It’s hard work to turn over a cold engine using the starter, and the battery chemistry does not perform as well when cold - ‘double whammy’
Assume the battery is good but something is draining it. Three factors will affect whether or not the car will start when tried 1) the length of time since it was last given a good run 2) the demand put on the battery when starting; how cold it is
It’s fairly easy to rule out, provided there are no problems caused by disconnecting the battery, radios etc.
I’m not an expert but I would have a go by putting an ammeter in the circuit at the battery, checking for drain and if present, pull fuses until it changed.
or
Give the car a good drive. Disconnect the battery, ruling out any possibility of a drain. Leave for a week. On a cold morning, reconnect the battery and try to start.
You can measure the voltage before/after too to add weight to the tests