Creating polygons with fixed extent based on point which represents S/W corner using QGIS

In QGIS, use "Geometry by Expression" tool (in a projected CRS). It will also add points information/attributes to polygons.

• Select "Polygon" as "Output geometry type" option.

• Add the following expression as "Geometry expression" value:

  make_square(make_point($x, $y), make_point($x+100, $y+100))
  

Using a Virtual layer, you can create a polygon of the desired dimension and move it to each location of the point layer. You can then save as the output if you want to persist the data.

Go to the menu Layer > Add Layer > Add/Edit Virtual Layer... and enter the following query. Replace the layer name for yours.

WITH src AS (
SELECT ST_GeomFromText('POLYGON((0 0, 100 0, 100 100, 0 100, 0 0))') AS geometry
)

SELECT ID, ST_Translate(src.geometry, st_x(pt.geometry), st_y(pt.geometry),0) as geometry
FROM "myPointLayer" pt, src

For each point (x,y) in a projected CRS generate a string like:

polygon((x y, x+100, y, x+100, y+100, x y+100, x y)) 

you can then process that into a Polygon using geom_from_wkt.

Then save the layer as a shapefile (or better still a GeoPackage).