Find the definition of these Widgets in the XML file defining the Templates:

https://github.com/odoo/odoo/blob/11.0/odoo/addons/base/ir/ir_qweb.xml

<odoo>
<template id="contact_name">
    <div t-if="'name' in fields">
        <t t-if="object.name">
            <span itemprop="name" t-esc="name"/>
        </t>
        <t t-if="not object.name and object.parent_name">
            <span itemprop="name" t-esc="object.parent_name"/>
        </t>
        <t t-if="options.get('country_image') and 'country_id' in fields and object.country_id and object.country_id.image">
            <span t-field="object.country_id.image" t-options='{"widget": "image", "class": "country_flag"}'/>
        </t>
    </div>
</template>
<template id="contact">
    <address t-ignore="true" class="mb0" itemscope="itemscope" itemtype="http://schema.org/Organization">
        <div t-if="not (('name' in fields) or (address and 'address' in fields) or (city and 'city' in fields) or (mobile and 'mobile' in fields) or (website and 'website' in fields) or (email and 'email' in fields))" class="css_non_editable_mode_hidden">
            --<span class="text-muted" t-esc="name"/>--
        </div>
        <t t-if="object.country_id.name_position != 'after'">
            <t t-call="base.contact_name"/>
        </t>
        <div itemprop="address" itemscope="itemscope" itemtype="http://schema.org/PostalAddress">
            <div t-if="address and 'address' in fields">
                <i t-if="not options.get('no_marker')" class='fa fa-map-marker fa-fw'/> <span itemprop="streetAddress" t-raw="address.replace('\n', options.get('no_tag_br') and ', ' or ('&lt;br/&gt;%s' % ('' if options.get('no_marker') else '&amp;nbsp; &amp;nbsp; ')))"/>
            </div>
            <div t-if="city and 'city' in fields">
                <i t-if="not options.get('no_marker')" class='fa fa-map-marker fa-fw'/>
                <span itemprop="addressLocality" t-raw="city"/>,
                <span itemprop="addressCountry" t-raw="country_id"/>
            </div>
            <div t-if="phone and 'phone' in fields"><i t-if="not options.get('no_marker') or options.get('phone_icons')" class='fa fa-phone fa-fw'/> <span itemprop="telephone" t-esc="phone"/></div>
            <div t-if="mobile and 'mobile' in fields"><i t-if="not options.get('no_marker') or options.get('phone_icons')" class='fa fa-mobile fa-fw'/> <span itemprop="telephone" t-esc="mobile"/></div>
            <div t-if="fax and 'fax' in fields"><i t-if="not options.get('no_marker') or options.get('phone_icons')" class='fa fa-fax fa-fw'/> <span itemprop="faxNumber" t-esc="fax"/></div>
            <div t-if="website and 'website' in fields">
                <i t-if="not options.get('no_marker')" class='fa fa-globe'/>
                <a t-att-href="website and '%s%s' % ('http://' if '://' not in website else '',website)"><span itemprop="website" t-esc="website"/></a>
            </div>
            <div t-if="email and 'email' in fields"><i t-if="not options.get('no_marker')" class='fa fa-envelope fa-fw'/> <span itemprop="email" t-esc="email"/></div>
        </div>
        <t t-if="object.country_id and object.country_id.name_position == 'after'">
            <t t-call="base.contact_name"/>
        </t>
    </address>
</template>
</odoo>