How can i use custom filed values in Templates?

The function you need is get_post_meta()

https://developer.www.ads-software.com/reference/functions/get_post_meta/

An example usage is a template may be ( inside the loop )

echo esc_html( get_post_meta ( get_the_ID(), 'my_custom_fld', true ) );

get_the_ID() returns the current item ID in the loop, true means a single value.
esc_html escapes any html – if you WANT html then use wp_kses_post instead to clean any malicious scripts that may have been snuck into the meta

That works for classic themes, but in block themes things seem to be more complicated.

My bad – didn’t see the block theme point.

You could always write it as a shortcode, i *THINK* that would work?

I converted it to a plugin (untested – feed back welcome )

https://gist.github.com/alanef/87300ec1f8c46a040dee87e7faa44620