So on the web app you can use the ArcGIS Javascript API (https://developers.arcgis.com/javascript/) in order to create a map and the editing functionality.
The best way is to use an ArcGIS Enterprise in order to Enable and create a Geodatabase on the SQL Server (https://pro.arcgis.com/en/pro-app/tool-reference/data-management/enable-enterprise-geodatabase.htm) and create the appropriate web services (Feature services) for the web editing (https://enterprise.arcgis.com/en/server/latest/publish-services/windows/what-is-a-feature-service-.h...)
Finally on the Web App you can use the out of the box Editing Widget (https://developers.arcgis.com/javascript/3/sandbox/sandbox.html?sample=ed_simpletoolbar) or create custom editing tools (https://developers.arcgis.com/javascript/3/jssamples/ed_feature_creation.html)
The geometries stored directly to the Geodatabase (SQL Server in your case)
If your case is not to apply the ArcGIS Enterprise Solution on the system the only part you can use is the ArcGIS Javascript API to create the map and the editing tools, only for the drawing tools (https://developers.arcgis.com/javascript/3/jssamples/toolbar_draw.html).
After that from the result graphic you can get the geometry (https://developers.arcgis.com/javascript/3/jsapi/graphic-amd.html#geometry) and from the geometry (depends from the geometry type, point line, polygon) you can get the coordinates (vertices)
For point (https://developers.arcgis.com/javascript/3/jsapi/point-amd.html#x)
For Line (https://developers.arcgis.com/javascript/3/jsapi/polyline-amd.html#paths)
For Polygon (https://developers.arcgis.com/javascript/3/jsapi/polygon-amd.html#rings)
Also you can convert the geometry to Json.
A sample is here:
if (graphic.geometry.type==="point"){
graphic.geometry.x
graphic.geometry.y
…….
if (graphic.geometry.type==="polyline"){
var RG = graphic.paths[0]
var countVertex =0;
for (var im = 0, ilm = RG.length; im < ilm; im++)
{
countVertex = countVertex + 1;
var vertex = graphic.getPoint(0,im);
….
….
}
…..
…
if (graphic.geometry.type==="polygon"){
var RG = graphic. rings[0]
var countVertex =0;
for (var im = 0, ilm = RG. length-1; im < ilm; im++)
{
countVertex = countVertex + 1;
var vertex = graphic.getPoint(0,im);
….
….
}
…..
….
At this point you only have on the client side the graphic and the vertices. With out ArcGIS Enterprise depends to you to implement the way you have to store the data on the database
For this case see the Developer options you have in order to use the ArcGIS Javascript API https://developers.arcgis.com/