Understanding textile work means, learning to choose the most efficient method of joining fabrics together, that is, the one that gives the best result, in the shortest possible time.
A seam is a join between two pieces of fabric. There are various things to consider:
Finish - usually we want a flat invisible finish, but that is not all.
Treatment of turnings - A seam should be done 1.5cm from the edge of the fabric for strength. The "raw" unfinished edges ,ust not show, must not be bulky and must not "fray" during wear. (Fray means the edges come unravelled.)
Fabric - What is it like? Does it stretch? Does it fray badly? Is it very thick and bulky? Is it very fine?
Fullness - Are the two pieces pf fabric to be joined the same size? If not how can the extra fabric be treated and worked into the seam and then neatened?
Pull - Will the seam have a lot of strain on it? Will one row of stitching be enough?
Wear - Is the seam subject to a lot of wear and tear?
Speed - Sometimes a machine can join a seam and neaten in one process, speeding up the construction process.
In most cases only the first two in this list apply, but the other points all call for different methods. The most common seam is an open seam where the seam allowances are pressed open after sewing and then neatened with a zig zag finish separately.
Here is a very old flow chart I drew up to help you choose what kind of seam to use. TFLx