How can I improve hair density my both temple corners hair thinning and looking frizzy?

I'd suggest seeing a hair specialist to review your exact diagnosis and to discuss treatment.  The most likely diagnoses for someone with a story similar to yours is early genetic hair loss as well as hairline maturation. See your physician to confirm and to review all the options.

Your biggest enemy is the progressive thinning leading to a
balding pattern that is not evident at this point in time. The degree of
thinning that you are experiencing would suggest that you should see a
doctor who most likely will get Bulk Measurements of your hair to
determine the degree of the thinning that you are presently having. If the
diagnosis is early genetic hair loss (the most common cause of hair loss
in young men), then the best treatment would be the drug finasteride which
is highly effective in men of your age and may not only stop the hair loss
but possibly reverse it. If you are developing a more advanced balding
pattern the best test to get will determine the Bulk Measurements of your
hair and this will show, over a one-year time frame, how effective the
treatment will be. Once you know this information, then you and your
doctor need to develop a MASTER PLAN which will define what you need to do
over the years to come.