I have been told at the stores that I should not strain the wort as it goes in the primary fermentor.
Since you're brewing extract you won't have much hot and cold break in your wort, but you will have some hop pulp, unless they're bagged.
Are you doing full volume boils or do you top up in the fermenter?
If you don't do full volume boils, (so you're topping up in the fermenter), you have very high gravity wort in your kettle, so any wort left behind multiplies to more lost beer in the end.
Either way, if you can strain tightly (like through a fine sieve or a mesh bag) then the wort loss would be far less. Just sure everything touching your
chilled wort is sanitized, such as sieves, strainers, bags etc.
One store says never strain till it goes in the secondary
Using secondaries is pretty much moot anyway, creating problems (oxidation, infection) while preventing none. Except in some very special circumstances (sours, fruit, long term bulk aging, etc.), I'd avoid using secondaries like the plague.
If and when racking to a secondary (when using one is absolutely needed) you'd leave the trub on the bottom, undisturbed, so there is no need to strain anything.
You'd start racking
from the middle of the primary, halfway between the beer level and the trub layer.* Then lowering the racking cane/siphon slowly as the beer level drops. Toward the end, you could tilt the fermenter slowly to keep the siphoning well deeper. As soon as trub moves up the cane, you stop the transfer. Also, put one of those flow diverter tippies on the bottom of the cane, to prevent or reduce sucking up trub.
* Main point: Don't stick the cane all the way on the bottom, in the trub layer, as seen in so many videos.
Similar logic and process when transferring your beer from the (primary) fermenter to a keg.
In either case you want to prevent any oxygen ingress after pitching yeast and the initial oxygenation/aeration. For that reason (semi) closed transfers into 100% CO2 pre-purged kegs is recommended.
