This radio does not have "subtrims". Those are stored settings that are recorded and applied even though the trim levers themselves are centered, in more advance radios.

One can leave the trim levers as they have been positioned for neutral flight, but if more than a bit has been applied, it will "unbalance" things a little. Likely not a big issue, but you may find differences in how it behaves in left vs right turns, or climbing vs diving. If you get another BNF plane that this TX can be used with, the trims will have to be redone for that plane - and your current settings for this one will then be lost. Or, you may accidentally move the trims, and not remember where they were (or may not notice they moved without the visual confirmation of "are they centered?"). So good to have the settings saved by adjusting the linkages in the plane, and always keep the trims neutral.

Because the gyro is trying hard to keep the plane level in the easier flight modes, trim lever movement will have reduced effect - what you put in will be compensated for by the gyro. This can, sometimes, cause one to overshoot the amount needed, and not know until the next time Experienced mode is chosen. Hence the instruction to trim when in Experienced mode in the air - so you get the correct amount needed.
Yes, afterward on the ground, put the plane into Experienced mode, with the trim levers positioned as they were in the air. Note the amount of up/down or right/left the trimmed control surfaces are deflected. Center the trim lever, and loosen the screw holding the pushrod to the servo. Move it as needed to get the deflection you noted earlier. Repeat as needed for the other channels.
You may not get it perfect the first time, or even the first couple of times, so take it up (any mode) and when straight and level, flip to Experienced mode and again trim as needed to keep it straight and level. Repeat the on the ground adjusting as needed.
The goal is to maintain hands off straight and level in Experienced mode (at 50% throttle) just like in the other modes. So if you have to flip in and out of Experienced (or use Panic) a number of times as you adjust the trim levers in flight, so be it. It is far easier to do on very calm days - wind in Experienced mode will make things harder.