Review of UML class Diagram of a ASP.NET Core Web API

I've set up a UML class diagram of my ASP.NET Core Web API that uses Entity Framework, with Models and Controllers

I would appreciate any feedback

Things i'm concerned about:

1. Multiplicity
2. Relations

Here's an example of the Bookmark.cs (model):

public class Bookmark
    {
        public int Id { get; set; }
        public int PostID { get; set; }
        public int UserID { get; set; }

        public virtual Post Post { get; set; }
        public virtual User User { get; set; }

    }

And it's controller (BookmarksController.cs):

[Route("api/[controller]")]
    [ApiController]
    public class BookmarksController : ControllerBase
    {
        private readonly WebContext _context;

        public BookmarksController(WebContext context)
        {
            _context = context;
        }

        // GET: api/Bookmarks
        [HttpGet]
        public async Task<ActionResult<IEnumerable<Bookmark>>> GetBookmarks()
        {
            return await _context.Bookmarks.ToListAsync();
        }

        // GET: api/Bookmarks/5
        [HttpGet("{id}")]
        public async Task<ActionResult<Bookmark>> GetBookmark(int id)
        {
            var bookmark = await _context.Bookmarks.FindAsync(id);

            if (bookmark == null)
            {
                return NotFound();
            }

            return bookmark;
        }

    
        // POST: api/Bookmarks
        [HttpPost]
        public async Task<ActionResult<Bookmark>> PostBookmark([FromForm]Bookmark bookmark)
        {
            _context.Bookmarks.Add(bookmark);
            await _context.SaveChangesAsync();

            return CreatedAtAction("GetBookmark", new { id = bookmark.Id }, bookmark);
        }

        // DELETE: api/Bookmarks/5
        [HttpDelete("{id}")]
        public async Task<IActionResult> DeleteBookmark(int id)
        {
            var bookmark = await _context.Bookmarks.FindAsync(id);
            if (bookmark == null)
            {
                return NotFound();
            }

            _context.Bookmarks.Remove(bookmark);
            await _context.SaveChangesAsync();

            return NoContent();
        }

    }

I don't know the domain, so IMO just some feedbacks about UML syntax:

• Aggregation arrow should have diamond on container (not on contained class).

• Higher abstractions should be on higher part of the diagram so inheritance arrows should be from bottom to top. It helps to understand the most important part of the diagram.

• If you can, avoid crossing arrows. You should try to divide your domain not to intertwine.

• Out of diagram scope classes should be grayed or diagram should indicate that they come from a different package (see ControllerBase and DbContext). It's usefull to understand dependencies.

• Class diagram should state extension points of the architecture. For instance, in the source code of Bookmark class, I saw two virtual properties. If your intent is to extend this class, you should indicate it in the diagram. At least with a dummy concrete class example. Usually I use color for it (even if it's out of the standard UML).